I suppose it's a tax management tool for the Gates, besides being an excellent charity. I don't mean to take away at all from the good they do, but why else wouldn't they accept donations except to not muddy the tax waters?
Because then people will use the "name that gets associated with Bill Gates upon donating", to further their own cause and popularity. Besides, they are rich enough to sustain their charity's causes, I am pretty sure they know about budgeting.