Because handbook says a certain amount is the maximum they can offer, and likely the crew offering more would risk their job. Following this fiasco, Munoz said they will expand the handbook. We'll see.
As in, among other reasons, is United in the wrong here because the maximum they offered for volunteers was $800, not the regulated $1350?
So bumping the person that paid the least for their ticket would mean they have to pay them the least.
`min(2 * your ticket price, $675)`; or
`min(4 * your ticket price, $1350)`
Judging by personal anecdotes I've read around the web, airlines may be willing to pay more if they are in a bind. But note that this whole voluntary/involuntary bumping procedure happens before you scan your ticket and board the airplane.