Remagen Presentation

I found the presentation very interesting. It is hard to believe that the Germans tried to blow the bridge and it didn't blow up and stayed standing for a couple weeks. The thing I don't understand is the Germans knew the Allies were coming and they didn' t have enough troops to stop them why didn't they blow the bridge earlier. I know bridges are important but Germany couldn't mount a counter attack at that time so the bridge was more important to the Allies than the Germans. So why wait and let the Allies take the bridge? This would have slowed the Ally advance for a little bit and maybe the Germans could have regrouped. I just wonder why people wait to blow bridges so long even if they don't need them.