Why are there so many superdelegates?
Given the chart you posted, I assume you're asking specifically about the Democrats. (Note that Republicans have a similar system, but use the term unpledged delegate instead of superdelegate. Both parties have roughly 25% of the delegates set up as this super/unpledged status)
The reason the DNC has so many superdelegates is because that's how they set up. Wikipedia gives a good summary of the history.
the Hunt Commission recommended and the Democratic National Committee adopted a rule that set aside some delegate slots for Democratic members of Congress and for state party chairs and vice chairs. Under the original Hunt plan, superdelegates were 30% of all delegates, but when it was finally implemented for the 1984 election, they were 14%. The number has steadily increased, and today they are approximately 20%.
To answer your other assumption in your comments:
I thought superdelegates were by definition unpledged
No--at least not in terms of DNC terminology. A superdelegate simply means they are a delegate chosen by the Democratic Party directly. When and whether or not they pledge is up to each one individually. The chart in your answer is merely pointing out the superdelegates that have pledged thus far. (As well as the regular delegates that were chosen for each candidate thus far).
However, as stated earlier, under GOP terminology, you could consider a superdelegate the same as an unpledged delegate.
Clear as mud? :)