Why couldn't he get it up?

It could be a number of things. Maybe he was nervous because he knew you hadn't done it before. Or maybe he just wasn't having a great day, and the timing wasn't great for him.
Next time, maybe have some KY liquid ready. If it had anything to do with you, that might help. Good luck. ;-)

Some guys just have bad days. I know I've had them. There are days where everything goes great and there are days where you just can't get really excited. Maybe that's the side effect of having a life besides sex. Sometimes, a guy gets a lot on his mind and just can't get it up because of that.

Maybe he has a problem or you might need lube before you do it. Some girls just don't get that wet for some reason. Maybe you did not have enough foreplay to get wet.