For me the main reason is that it is quite hard to convert it to fiat. Talking about that, I find them too greedy. They're doing more and more fund raising campaigns, and I don't like that pretty much.
They run the 7th most popular website in the world,
totally free of charge- no ads or commercials,
for one of the biggest sources of knowledge in the free world.
I hardly think that asking for a few dollars in donations from each user is 'greedy'.