He actually was close to signing but had second thoughts about pissing off Sheen, from what I understand.
Kutcher doesn't care, nor shouldn't. By the time the show actually ends, Charlie will probably be dead from god only knows what self-inflicted thing he brought on himself.
I had read at one point that even Jerry O'Connell was up for the role, dependent on whether or not his show with Belushi was going to get another season. They could have just said it was Charlie Sheen from another dimension, and he's stuck there for 29.7 years until the next slide window opens.