Hillary still had a primary to deal with for the first week of June, the party was already united behind Obama. Also, I'm not sure whether this is true or just BS (I lean toward the latter), but I remember reading that many Democrats are holding off on donating until after the RNC, since they don't want to intimidate them so much that they dump Trump at the convention.
Ah, fair point. Forgot about the primary issue. And yes, I probably read the same thing as you - I do hope they are just holding off, because this cycle is truly a rare gift to Democrats and Clinton / DNC should really go all in. We probably won't get another chance like this for some time.