Because Google has a monopoly on non-shitty free email with GMail, in the same way that Microsoft has (had?) a monopoly on hardware-independent operating systems with DOS/Windows. (I wish I was really being sarcastic, you know because there are actually a billion web-based e-mail alternatives besides GMail that actually don't suck... right? Anybody know of any? At this point I'm earnestly inquiring.)
Gmail might be 'the best', but not enough to get a everyone to switch. So, IMO it's a question of where your suck threshold than anything specifically wrong with Hotmail / yahoo.