Does it happen every year? Can he go next year?

Seriously, like someone else said, there are only so many things one can do in 24hrs.

He may be president, but who ever does his scheduling, I'm pretty sure you have to go by that schedule since that's how they know where he will be and know where to have security. He can't just say "Oh, I think I'll just skip this and go where I want to go." Things are organized very specifically when you're in a public office and need security.

It's not like he just stopped and got pizza instead.