The DraftExpress article does the best job of explaining what is happening. What the NCAA has done is basically incentivize players to wait until the end of April to do anything. They can't work out until May, so those on the fence will ask for the evaluation from the NBA (which doesn't equate to declaring), and will wait until the NBA entry deadline to make a decision -- factoring in who is leaving, who is staying, etc. So a guy like Mason, who may enter the draft depending on who else does, benefits by waiting as long as he can to see (1) how he is evaluated, (2) how other early entry players are evaluated vis a vis Mason, and (3) who that may have been ahead of him has decided to stay in college.
All the NCAA did by moving up the withdrawal deadline is to, as others have said, eliminate testing the waters. It also incentivize players to wait until the NBA deadline to make a decision, thus mucking up the rosters of their former college teams who are trying to juggle scholarships themselves.