Because Stern is selling to the mass market and the mass market has settled on what the standard bottom should be. I'm surprised if I see more than one inlane. Plus I'm sure operators rather have outlanes drain than be infinitely savable.
It's one of the reasons why I love my Space Odyssey in my collection. It at least does something very different than every other game in my collection. (And guests who aren't into pinball love its variation on slings)