I doubt it'd be because of a security reason of some sort. More likely to be because people don't see that they have value or something along those lines.
Gmail is more secure than most others, so they're not taken and sold here as often. AOL emails are everywhere. Most are just sold for AIM, though, so you probably haven't noticed.
Gmail is secure as fuck. As they where made from the godfathers of the internet. Google.
You'd most likely see some gmails once a year or so and most of them tend to be shit.
but when there are good ones you take em asap but they could be worth like 80+
aols have no value
Gmail a aren't really sold cause you cannot really hijack them, especially with the new feature where to change recovery email, the old one will still be there for 30 days.