Why didn't the Great Migration begin 50 years earlier?

[(Still) muted by Kalwejt until March 31]

It doesn't make much sense to me. Vagrancy laws in the North? I don't get it. Massachusetts had much higher wages than Mississippi in the 1870s, and had a population much less hostile to Blacks. How come Massachusetts wasn't outright majority Black by 1910, then? Instead, Mississippi, despite being solidly White racist-ruled, was peak %age Black in 1910.

[True Federalist (진정한 연방 주의자)]

You had ties of kith and kin to keep you home and no prospect of doing better economically if you did engage in the arduous task of moving. It wasn't until World War I that economic opportunities became starkly better for those moving north. Even that would likely have been a short term phenomenon were it not for the cut off of external immigration.

[Beet]

There were actually two Great Migrations. The one from 1910-1930 was relatively small actually, the one from 1940 to 1970 was far bigger. Basically, it wasn't until the mid-20th century that the northern situation for the average laborer became far better than in the South. Ironically, this was not the peak of per capita GDP differential between the North and South, which came earlier. However, this was the time at which northern laborers had the best conditions, due to the union movement. It is improved conditions for workers that draw migrants in, and the differential must be large to encourage the risk.