Imagine if he missed this very important service because he was halfway across the world celebrating some Germany/Russia thing from 20 years ago.
Sounds like he made the right call skipping that anniversary event? Sure, it means a lot in the historical sense... but Americans need to see their leader at the memorial for this very recent attack, which may be classified as some in the Senate (as well as many americans) as a terror attack.
Imagine if Bush had skipped visiting Ground Zero, and instead visited Germany to talk about something from the 80s. Nope. The nation needed their leader there for the healing process.