The same reason you don't see many central drop target banks: they require unsafe shots. With today's super fast flippers it's hard to make it worthwhile. Why shoot drops or rollovers when you can hit a ramp? Can't have a game without ramps!
Quoted from Paul_from_Gilroy:
I owned a WOZ for 3 years. In my opinion, the rollover buttons in WOZ are novel, but not well done in terms of fun. Their placement is such that, most often, you hit them inadvertently. That's fine, but they are an important part of the rule set (you need to hit them to spell out the characters, which is part of making progress in the game). I'm not a game designer but it seems to me that's not a recipe for fun (when the things you need in order to make progress are the things you usually hit unintentionally).
The problem is, if you out them in front of a shot, then it's the same as shooting a shot. They don't really make much sense unless you have to aim for them.
It could be argued that the whole idea is to make it so you can't just go for multiball all day without being in danger, but if you ignore them for other parts of the game, you'll progress just by accident. Same with all the rainbow targets, you don't really aim for them unless there's maybe one or two left. Or any of the lower side 2/3 banks of stand ups on modern fan layouts.
One cool use of rollovers that could still work is having them hug the wall on orbit shots like on some ems (Little Chief, Delta Queen for instance). The idea is that, shooting the orbit itself is pretty easy, since it'd wide and you can just bounce off the wall and make it up, but if you want the extra points you need to shoot the wall right at the start and 'hug the wall' as the ball goes up. A good shot will hit all three rollovers for max points, but it's more risky, and you can also try to hit just one or two for a partial risk.