Of course, this is just a guess, but I have a feeling it is no longer vital nor profitable to have live news updates in the middle of the night. I remember in the 1980s when there was the great Larry Glick, it was a very different time. Much of Boston wasn't wired for Cable, and there wasn't much on TV in those hours (if there were any stations on, I remember only WBZ TV being on all night long). But now, with 200 + channels, I bet people are just not up listening to 'ole WBZ, which probably means they aren't selling ads at that hour, or for a good rate, which probably doesn't make it logistical to keep a paid staff working all night.
Consider this: Out here in West Texas, recording news breaks during the day have been routine for YEARS. In the overnight, they don't even bother, and just take feed from the networks atop the hour. And why not? If you know that you can get live updates from CBS, ABC and Fox, who needs local updates, unless there's an emergency?
I know I sound like an apologist, but I have to think that with all the money that the broadcasting companies are losing, they can't afford (or be expected) to bankroll the handful of us radio enthusasiasts awake at 2am.
I wish this what the old days, but it's not, times have changed