As long as either side thinks they can still gain a military advantage, they will do so before starting negotiations. Putin doesn't seem to be open to negotiations with reasonable conditions anyway. The bridge is also a major military supply line and therefore a legitimate target; it is a sad truth of any war that civilians will also suffer.

I am a bit surprised that they do it now, though; I would have expected it a bit later, at a moment that Ukraine would be strong enough to reconquer Crimea.