They had trouble with simultaneous releases when they put out 4e, there were some troublesome proofreading/quality issues. So with 5e, they put out the pieces one at a time, allowing each title to have its own turn to be the urgent, top priority.

I started running 5e before the release of the Monster Manual 5e, using the smattering of monsters in the back of the PH. It was limiting, but fun in its way.