Probably some combination of:

a) He's getting asked a lot.

b) By publicly announcing a deadline, everyone involved (including him) will be racing to hit that deadline or lose face.

c) He's telling the world "I say what I do and I do what I say".

There was that recent PR article that made the rounds here last week from some company allegedly developing an evacuated tube based system that was written in a way to imply they were working on Musk's idea. I would not at all be surprised if he's a little miffed about that and has decided to just get everything out there.

Given that they may not actually be working on what he specifically has in mind, there is a risk that his idea gets a bad connotation if someone implements something similar (but not identical) and does it poorly. There is a whole political dimension to this in terms of getting zoning, approvals, etc... and the last thing he needs is for it to be stillborn because a bunch of politicians hear it "was tried and failed".

Maybe to apply social pressure to himself to get it done by that date.

It also helps that two major accidents happened recently.