It's all the little traditions that are supposed to keep the President in the people's favor. What if Obama decided to kill and eat the turkey that he was supposed to pardon on Thanksgiving. First of all, I think that would be funny as hell, but there would still be an uproar from the people, peta and animal rights activists come to mind. In this case, nobody would have said anything if he had just gone to Arlington. I'm sure he meant to do right by going to the veteran's cemetery in Illinois, but why break tradition? It makes no sense why he can't just go to Arlington, lay the wreath then go Illinois. Everyone cares about what the President does, because he is the leader of our country, he needs to make more of an effort than the regular politician.
As for the other Presidents that failed to go to Arlington, I'm sure there was a similar response from the people as well.