Because one can create (theoretically) unlimited gmails with a single account. Hotmail/Live/MSN account creation is heavily troubling with the JS and captcha to create a single account unless you have the right software to automate the creation process easily.
But the thing is, if you create 100,00 Gmail emails such as [email protected], [email protected] (I assume this is the 'gmail trick' as it's really the same email, just a bit added to it), Twitter can spot this and so doesn't just ban 1 account, it bans all 10,000 of them in 1 foul swoop.
I'm only going off what has been said throughout BHW and on other threads, but people seem to be using Gmail, so perhaps this isn't always the case.