I myself was mostly in Community Chat, so an even less Lego-related forum than what you were familiar with.

The MBs weren't the only thing they closed in that time. The gallery soon followed. I suspect the overall reason is that they didn't want to keep dedicating resources to that community, they probably felt they weren't getting enough return. Probably true, though I know for myself my interest in Lego waned after those communities were gone. I had no incentive to keep building stuff.