uhmm... if I remember correctly we turned if off after some heavy trolling and spamming of users through it, I hope qudobup remembers more clearly... was a few years ago.
Given that PMs would be for private communication, and that can be accomplished using email, it was decided that the PM system is redundant.
That was the main reason. (It was Qubodup's decision and I can not fault his logic. If you can give a solid reason for restoring the PM feature, then we will consider it.)