joke/riddle

there is a 3 story apartment building. on the first floor, only black people live there. on the second floor, only hispanic people. and on the third, only whites. it ends up that the building isnt structually stable and collapses killing all of the blacks and hispanics, but no whites. why?

if you have heard it before just comment that you already heard it. everyone guess please. 1 guess per person! reward 4 winning most likely =D