Because why try to make a change when you can just give up and quit? /s
Bigotry isn't a part of the Republican platform, it's a disease that members of the party have and isn't going to be changed until those people start getting pushed out from within.
..and before you try to argue that it's permanently engrained remember that the Democratic party once supported slavery and the Republican party was formed in direct opposition. So things very much can change.
It takes time to change the hivemind and it can only be done by people standing up against it from within, not from people surrendering and fleeing.