And what problem would that be?
Ranges anywhere from it's boring to it's bad to it's pointless to it's a wasted opportunity.
Watchmen isn't untouchable and there's no reason not to revisit it if you can create some really great stories out of it. But nothing about Before Watchmen is really great, and only a couple of the books are even any good at all.
Basically, it's not good enough to warrant having been made in the first place.