I'm guessing it's either because of scope or because generational collectors are a type of copying collector. Instead of having just from/to spaces that the objects ping-pong between as they fill up, a generational collector will copy objects from one heap generation to the next as they fill. Once the oldest generation (tenured space) reaches capacity it cannot trickle down the live objects to another generation so some other method, like a mark+sweep+compacting method, is used for collecting it. You could probably have a half space tenured generation but half space collectors are not really space-efficient without resorting to a bunch of virtual memory mapping trickery, and the tenured space tends to be large.
