Many of the totally unskilled people that immigrated to North America (Canada and the USA) in the 19th Century became farmers, or were able to find low-skilled (or even "no skilled") labor jobs simply because industrialization processes were not even close to where they are today, so the best option was to get as many people over here as possible and just "throw bodies at the problem" to meet the desired/required quotas to keep the nation fed and moving forward industrially/technologically.

The nation needed to be fed for example, and the best way to do that was to find people willing to work their asses off doing tough manual labor, because after all, it did not require tremendously skilled or educated workers to clear and till and work the farm fields, planting and watering crops and harvesting them, or to work 10-12 hour shifts in factories doing assorted back-breaking tasks that any uneducated person, many of whom were not even fluent in the local language, could and would be willing to do. It just required a lot of hard, albeit largely unskilled, work by people willing to put in the tough labor to make that happen. Joining into an agrarian/farming workforce was an easy route to both immigration and land ownership back then, with nations like Canada (my home country) seeing its citizens being fed on the backs of a lot of immigrant farmers, who were actually given plots of land to own, live and farm on upon immigrating here in places like Alberta and Saskatchewan and Ontario by the government in exchange for them clearing and working the land and growing crops there.

Of course, there were also skilled entrepreneurs immigrating as well who did their own thing, starting their own businesses or working for already established ones due to the skillsets they already possessed (master woodworkers and various craftsmen for example), but a large bulk of the initial unskilled immigrant workforce coming over were required for the nation to grow and prosper due to their ability to, although largely unskilled, work hard at labor-intensive jobs like farming and factory work, and thus were granted citizenship quickly and the farmers in particular were able to easily acquire their own patches of open assigned land in the prairies to begin cultivating and farming, being handed that land by the government. After all, one thing we in Canada have a hell of a lot of, particularly in the "Prairie provinces", is arable land suitable for growing crops. My distant relatives generations ago for example immigrated to Canada from France (on my Mom's side) and Ukraine (my Dad's side) and were initially farmers in Alberta, taking advantage of the government land allotment offerings in exchange for helping to feed the country and help grow the economy.

Now however, with drastic improvements in things like farming cultivation techniques and modernization/technological improvement methods that dramatically cut down on manpower/human muscle requirements in the industrial sector, as well as the constant tech improvements in virtually every sector of the workplace and economy and the large growth of an educated populace, the immigration dynamic has shifted to the desire for a modern nation wanting immigrants who can bring more intellectual and practical skills to the table rather than just bringing an uneducated strong back and muscles and a desire to work hard. This shift was somewhat gradual, taking place over a great many years throughout the 20th Century, so I don't think it can be pinned down to a very specific time period. The industrial and technological revolution, which steadily cut down (and is still in motion and cutting down to this day) on the requirements for human muscle-based workload demands, is what has brought the steady change about, where we now see a demand for a largely more skilled labor force of immigrants as being more valuable and in demand than an unskilled group who only had raw muscle power and a willingness to use it as their predominant skill factor they were bringing to the table.