The DNC is a private entity, subject to its own internal regulations and policy-making procedures.  Its overall mission is to further the aims of the party, which includes decisions about allocating the party's support in an election year.  It's precisely because the Republican Party lacked an effective policy-making core of professionals (Reine Priebus and his pathetic crew being good mostly as straight men for the media) that they're stuck with Trump this time around.