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".

is this the company you are referring to? http://www.et3.com/

Well those guys are pretty transparent eh, a google checkout for their license http://et3.net/book/et3-license

I get physically ill trying to empathize with intellectual property sycophants.

Yes, here's the relevant discussion from HN:

https://news.ycombinator.com/item?id=6018064

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

It also helps that two major accidents happened recently.