Yahoo are still huge. I don't know if Gmail's passed them in users yet, but as of a year or so back, on a large (and admittedly old) general corpus, Yahoo was at least 2x larger than the next largest service provider.

Given technical debt and other management issues, they're a somewhat appealing target (lots of payoff, possibly easier to hack than better-managed systems).

Not that other providers haven't been hacked as well.

Could be that the typical Yahoo mail user is less technically savvy than the users of other email providers and more prone to making mistakes which makes it easier for the account to be hacked.

Dunno, Hotmail used to get hacked all the time.