Because the median voter theorem implies that elections will be roughly 50/50, and if minorities are more Democratic, then someone has to be more Republican to counter them.
The reason most Republican white males go to the party is because of Republican image as tough guns that will kick your ass. I wouldn't care if this image was in the Dems party but it will get to the point when they will become sexist to ladies and homosexuals that will switch me to third party.
Eh, this might play a small role, but by far the biggest factor is surely that white males are typically more privileged and face less discrimination than members of other groups. The only way that Democrats could win them barring a landslide is white males stop being the majority group or Democrats start appealing to majority groups (in which case they'd probably need to shift to the right of the Republicans).
