The Romanians explain in BD that “petrification” of the skin and eyes of vampires can occur if they remain still and wrap themselves in their own thoughts for very long periods of time. That effect can be seen not only on the wives but their husbands as well, so staying in the tower isn’t something that just the wives do. I see the husbands as leaving the tower more often, but not frequently, maybe once every several decades. None of them have any need to hunt, they have their food brought to them, so there isn’t much reason for them to do much of anything, if they don’t want to. I tend to take that as a subtle metaphor for moral decay, but that is another discussion.
I think that once the petrification settles in, it also makes it much harder for them to intermingle with humans, as their skin appears unusual even when they are not directly in the sun. They probably view staying in the tower as just being more prudent, and eventually settle into the habit so firmly that most vampires cannot recall a time when they ever left the tower.
"It will take an amazing amount of control,” she mused. “More even than Carlisle has. He may be just strong enough…the only thing he’s not strong enough to do is stay away from her. That’s a lost cause.”