According to this thread, a Superdelegate with inside info says that many of them are freshmen facing tough races and don't want to commit too early. Says they'll probably go with the Pledged Delegate winner, (so-called "Pelosi Club").
But there's also such a thing as committing too late.
For the sake of the image of the Democratic Party, of course, the DNC would probably like to see most of the remaining Supers endorse BEFORE the last primaries are held so that it will appear that the PEOPLE made the final decision to choose one candidate over the other -- not just a few superdelegates.
But the Superdelegates themselves probably have a much more selfish reason for not wanting to wait too long. Put yourself in the place of a superdelegate for a moment -- would you want the spotlight of the entire nation to be on you when it gets down to the last few delegates that either candidate needs? Would you want to deal the hate mail and vilification that would come from either side when you make your choice? Could you easily deal with that kind of pressure being placed upon you? Especially if you're not already a public figure?
For this reason, once Obama gains more than 50% of the pledged delegates, I think you're gonna see not just a trickle but a landslide as they fall all over themselves to ensure that they're not out there alone ready for the media to pounce on them and say, "Here's is one of the LAST Superdelegates, who will he/she vote for?"