I know i'm a bit late now that the bridge seems to be collapsing, but:
1. Anything worth blowing up is worth defending, meaning Russia has to put resources aside 24/7 to defend the bridge, which means they cannot aid any offensive action
2. If Ukraine ever chooses to invade Crimea, the bridge is an easy escape route for russian soldiers.
Sun tzu, yadayada, give your enemy an escape route, because then they are more likely to run than fight to their death