Europeans have to research Immigrants at the Capitol, a building that can only be built in the Imperial Age. In contrast, Native Americans and Asians can research it at their Town Centers in the Industrial Age. Africans can research Strangers’ Quarters, a similar technology, at TCs in the Industrial Age as well.
Is there any reason for this difference?
I think the only reason is, because the Capitol is an imperial age building and nobody would build a Capitol just for immigrants … its a very expensive tech and its not that important, If you get it at age 4 or 5. There is no reason civs without Capitol should wait for it until age 5
The Capitol should definitely be an age 4 building but with most of its techs locked behind age 5. A few like Immigrants and Knighthood could be available in age 4.