With the exception of some areas in Louisiana and New England, there are virtually no large communities of French speakers in the United States. In contrast, French is the official language of Quebec.
There is a rather large independence movement in Quebec due to perceived cultural and political differences between them and the rest of Canada, going so far as to have a formal independence referendum in the 1990s where independence was defeated by barely 1%. In contrast, virtually all of the territories that were part of the Louisiana Purchase are thoroughly American. With the exception of some quirks of Louisiana state law, nobody would say there are meaningful differences brought about by their French history.
What led to this difference?
There's always more to be said, but you may find pertinent answers in a recent post of mine about Quebec and French Canada's historical struggle to retain and protect its unique identity in an English-dominated continent. You will also find more about Quebec's separatist movement in the link above.
After the British conquest of Quebec in 1759, the victors realized that they were outnumbered in the colony 25 to 1 by French Catholics, concentrated in the St. Lawrence Valley. The expected migration of American colonists, which they had hoped would overwhelm French Canada, did not occur. Accommodation was seen as the best way to placate a hostile population in the tense years preceding the American Revolution. These accommodations were enshrined in British law with the 1774 Quebec Act. French language and culture were again protected under Canada's 1867 constitution when it became a country.
In contrast, Louisiana was ceded by France to Spain in secret in 1762. Outside of New Orleans and its environs, French settlements were sparse and scattered over an immense territory. Spain was an absentee landlord for much of next 30+ years. Driven by a plantation economy, Louisiana had a large population of slaves, free blacks, mulattos, white landowners and Acadian exiles. This diverse mingling of cultures created a much wider range of dialects blending French, Spanish and Creole.
While the Quebecois retained its cultural and linguistic identity under British rule, Louisiana's diversity would see its French language evolve and develop its own character -- but without the laws to protect it. France only regained Louisiana (briefly) under Napoleon before he sold it to the US for $15M. The American government pushed for the anglicization of Louisiana throughout the 19th century and into the early 20th century, despite its large French community. Over time, English became the dominant language in the state and cultural assimilation was normalized.
Although many Louisiana French shared a common ancestry with French Canada, their histories would diverge soon after the Seven Years War.
So a short answer is the majority of French descent colonists lived in the St-Lawerence river region (Canadiens) and to a lesser extent the Maritimes (Acadiens). The entire interior of New France was mostly trade outposts that traded with the local native confederations and communities. Louisiana had some more extensive French settlements and many of the deported Acadiens (see Le Grand Derangement) ended up there, but it was a much smaller population.
It's important to note the demographic differences between New France and the 13 Colonies were massive. France's colony was mostly economic and the population of New France was only about 70 000 people (1763) whereas the 13 Colonies had 2.5 million (1776). By the Louisiana purchase (1803) there seems to have only been about 30 000 Europeans and 30 000 enslaved Africans on the territory, in addition to all the First Nations people living in the interior.
A really important development after La Conquête and the Treaty of Paris (1763) was that the colony remained majority French and majority Catholic. Despite emmigration of the French elite and the immigration of English settlers, the catholic population remained considerable in the Saint Lawrence region. In an attempt to appease tension in the years leading up to the American Revolution, the Quebec Act was passed (1774) which protected the status of French, Catholicism and some French Laws. This was seen as one of the ''Intolerable Acts'' that contributed to the American Revolution. American Protestants were extremely uncomfortable with the protection of the rights of Catholics.
So french language rights became protected in the colony. A very precarious equilibrium was created and the entire political history of Lower Canada and Quebec from the 19th century to now is needed to explain how this equilibrium was maintained and fought for. It was 250 years of sustained political and social resistance. Needless to say that demographic weight was not present in other territories.
Some important points from that period though are that during the 19th century, early nationalists and the clergy encouraged natalism. This along with other sociological factors led to a massive population boom from the French Canadian population and those populations eventually moved around the continent. Large groups ended up in Ontario where there are half a million Franco-Ontarians today and likely millions of french canadian descent. Manitoba was settled by largely French speaking Métis populations who were descendants of different European traders, adventurers etc... Many French Canadians also moved to the American North East forming large communities in a number of industrial towns. Nationalists eventually grew concerned that the province was becoming depopulated and advocated for many to return.
The next important point, and this is likely the most important for the survival of languages was there was a considerable movement in the very late 19th century and well into the 20th century to ban access to French schools in Canadian provinces. This was successful in Manitoba with different laws passed between 1890 and 1916. This was also successful in Ontario in 1912 with a partial repeal in 1927. French Highschools in Ontario only returned in the 1970s.
Louisiana also had a similar history with access to French schools. It changed several times between 1860s and 1921 when all education was reverted to being exclusively in English. French education has returned to Louisiana but not until the 1970s.
So I'd say the three main reasons for the survival of large french speaking communities in Quebec: