I'm going to tackle the last 50 years or so of this answer. There's a lot to say about how Louisiana and Quebec got to where they were respectively in 1970 or so, but those points have a major impact on the language policies in those two polities.

First, even though there was a perceived decline in the French language in the 1960s, Quebec still was a majority-Francophone province. English-French Bilingualism accounted for some 30% of the adult population of Quebec in 1971. In 1981, there were 6.1 million Canadians listing French as their mother tongue, and the vast, vast majority of these lived in Quebec (which itself had a population of some 6.5 million).

By contrast, the best that I can find for Louisiana is that in the late 1960s, there were some million or so speakers of some dialect of French (more on this in a minute) out of a population of some 3.6 million. The Francophone communities in Louisana are furthermore heavily concentrated in the Southwestern part of the state. Shreveport is far, far away from Francophone Louisana, and even Baton Rouge isn't terribly close.

So to close for demographics, and if someone has more exact numbers please chime in, we're talking about Quebec at lowest being something like 75% Francophone in the late 1960s, and Louisana at highest maybe 30%.

Now specifically on language, that number from Louisiana hides some pretty stark divisions in community and language. While most French speakers in Quebec speak the same dialect, French speakers in Louisiana historically have been divided between Cajun (Acadian) speakers and French Creole speakers. As both communities distinguish themselves from the Anglophone majority and from each other based off of their dialects, this greatly hampers some sort of common language identity - you're not going to strengthen Cajun or Creole identity by teaching metropolitan Parisian French in schools or speaking it on TV, and so there isn't a "standard" that can be adopted in the same way that Quebec has. On top of this, speakers of French dialects in Louisiana can be white or black, and given the history of racial segregation in the state, racial identities had a real legal meaning dividing these speakers from one another - you would have black and white schools, for instance, so even if you had French-speaking schools, they'd have to account for this.

So finally, the big difference in the time frame I'm looking at is political and legal. Quebec had a resurgence in Francophone political identity in the 1960s, as part of its broader social, political and economic "Quiet Revolution", with the Parti Quebecois formed in 1968 to agitate for linguistic and political sovereignty of the province (you even had the more extreme FLQ commit acts of terrorism in the name of independence). The political revival resulted in such provincial legislation as the 1974 Official Language Act and the 1977 Charter on the French Language. The explicit goal of the latter is ""to make French the language of Government and the Law, as well as the normal and everyday language of work, instruction, communication, commerce and business." This means that courts, education, communications to workers and consumers, and government communications must be available in French.

In contrast, the legal mandate for French is much weaker. In 1968, the Cpuncil for the Development of French in Louisiana was founded, and its mission is to assist local school districts in the teaching of French (not providing full education in French as a first language). It uses metropolitan French and recruits Canadian, French and Belgian teachers, and so again is disconnected from local Francophone communities. Its powers are based on encouragements rather than the full force of the law, as in Quebec, and outside of educational programs it promotes such activities as encouraging localities to use French signage, and much of this is geared towards tourism promotion rather than encouraging a use of French in all spheres of daily life.

So in summary: when we're comparing Quebec and Louisiana over the past half century, we're looking at much different polities. Francophones in Quebec (mostly) speak and identify with one dialect of French, and make up an overwhelming majority of the province's population, and there is a strong base of political support to enforce (not just encourage) the use of French in governmental, media, educational and commercial spheres. Louisiana, by contrast, has a Francophone population that has long been divided by region, dialect and by race (which for much of modern Louisiana history has been a much more salient legal divide between Louisiana residents), and as such there is a much weaker political base for usage of the French language.

Before responding about the specific issues in Louisiana, I would (as an anthropologist) like to point out that while Language is very important for cultural retention it is only part of what we'd identify as a community's culture. In other words, there are a lot of French (and Spanish, Canary Islander, Native American, West African, Irish, Italian, German, Jewish, Basque, etc) traditions and threads of cultural heritage in Louisiana even if relatively few still speak fluent French.

So let's talk about French communities in Louisiana. First, it is important to recognize there is not just one French community/demographic in Louisiana. Very broadly speaking there were two - people who came directly from France and were a mix of lower nobility (often younger sons with no hope of inheriting much), merchants, and people of lower classes hoping for new opportunities (ex: casket girls.) Most of these Louisiana French (often called Creoles) were either in the countryside as planters and slave owning plantation owners or in New Orleans, which was the seat of political, economic, and social power. Then you had the Acadians who had originally settled in Acadia. Cajuns (also known as Acadians) were the peasant French who first moved to Acadia (modern day Nova Scotia) before being forcibly exiled by the English and settling in large numbers in the rural swamps of Louisiana between 1765 and 1785. By 1865, this group had split into the genteel elite Acadians and the lowly peasant Cajuns (Henry 1998.) Isolated and stigmatized by the French elite of New Orleans and Anglo newcomers, Cajuns continued to develop a unique culture separate from the rest of the state.

Then, in 1868 the so-called "Carpet Bag Constitution" outlawed French in Louisiana. This was part of reconstruction that was happening throughout the South after the Civil War. And to be clear there were some fantastic aspects of this new constitution including rights for the newly freed black populations in Louisiana. Those rights were strongly fought for by communities of color in the state (see this amazing poster) who were influenced by other successes in places like Haiti. But the French aspect of this was a huge shift. The constitutions of 1845 and 1852 required laws to be written in both English and French. By '64 that requirement was dropped but you still couldn't bar anyone from political office due to language and the constitution was published in French, German, and English. So in '68 this was a fairly big change as part of efforts to get Louisiana in-line with the rest of the country, which was at least at the higher political levels mostly Anglophone. In New Orleans, the shift in power and economic dynamics meant if you wanted to stay relevant you had to learn English to work with the influx of outsiders who were now politically and economically important.

But to some extent, Cajuns were far enough removed from the big city that they were able to retain a rich culture of art, folklore, music, language and dance that was transmitted orally through Cajun French from generation to generation. The 1879 state constitution reversed the requirement for primary school to all be in English, allowing Cajuns to attend public schools (when they could get to them) that taught children in their first language.

Then in the early 20th century, Louisiana undertook an aggressive effort to Anglicize Cajuns. They were forbidden from speaking Creole French in schools, encouraged to adopt more “American” last names, and to give up traditional lifeways. The state constitution of 1921 required all public schools to be in English and there were renewed efforts to get Cajuns into schools. One of the reasons they had been able to retain their culture and language was isolation but that isolation went hand in hand with poverty and lack of access to roads, electricity, or schools. In some areas this disconnect lasted as late as the 1960s. But in the 20s there was this effort to anglicize the nation, which followed on the heels of WWI and was common in many other areas of the nation. A shared unified and homogenous identity as Americans required selecting specific cultural traditions and connecting that with ancestry, language, appearance, etc. Anything that wasn't WASPy lost out in this cultural imaginary of American identity including French traditions in Louisiana. (If you want to learn more about Louisiana law and the French Language this review articleis great.)

However, that's not the end of the story. In the 1950s, a movement began within the Cajun community that rejected this Anglo-hegemony and reclaimed their culture as a source of pride. In 1968, Council for the Development of French in Louisiana (CODOFIL) was established to preserve and promote Cajun culture. Part of this included marketing Acadian lifeways to tourists. Cajun pride was further bolstered with the election of the initially very popular, though very corrupt, Cajun governor Edwin Edwards in 1972 who ended up serving four terms. Increased exposure led to regional and eventually national interest, which cities such as New Orleans and Lafayette successfully turned into tourist pulls. Cajun tours, villages, crafts, music, dancing, and food are now popular tourist attractions that generate large amounts of money for those regions. But as equally important, Cajuns are now proud of their ethnic and cultural heritage and little stigma remains. In 1976, South Louisiana audiences cheered as presidential candidate Ronald Reagan announced he had been appointed an, “honorary Cajun c**nass,” though some of the older generation winced at the term (it is an ethnic slur.) It seems that as the French elite and Anglo middle class began to realize the money making potential and academic interest in Cajun heritage they were willing to set aside age-old stereotypes and biases in order to make the most of this new opportunity. Today, we're seeing a modest revival of French language and attempts to retain some of the culture in concerted ways that actually have some funding.

One great resource that gets state funding is this website and set of resources on Louisiana Folklife: http://www.louisianafolklife.org/ . They have essays written by scholars (including some anthropologists that I know), photos, videos, maps, and lots of other great resources. If you want to explore you'll likely find some interesting ways that French culture has been retained in Louisiana not just through language.

So TLDR: there were specific legal and cultural efforts to Anglicize French cultural communities in Louisiana due to various political and social events such as Reconstruction and WWI.

Some additional readings:

Bernard, Shane K. 2003 The Cajuns: Americanization of a People. Jackson: University of Mississippi Press.

Brasseaux, Carl A. 1978 Acadian Education: From Cultural Isolation to Mainstream America. In The Cajuns: Essays on Their History and Culture. Glenn R. Conrad, ed. Pp. 212-224.

Henry, Jacques. 1998 Summer From “Acadien” to “Cajun” to “Cadien”: Ethnic Labelization and Construction of Identity. Journal of American Ethnic History 17(4):29-62.

Mattern, Mark. "Cajun music, cultural revival: Theorizing political action in popular music." Popular Music & Society 22.2 (1998): 31-48.

Estaville Jr, Lawrence E. "The Louisiana French language in the nineteenth century." Southeastern Geographer 30.2 (1990): 107-120.

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