Y'know what... I actually think that was a good idea and wish more companies would do this!
The people who were in to this game already from the first trailer were going to buy it almost Day 1, no matter what. So there was no need for a demo then, because the main audience they were selling to was already firmly convinced and on board.
Now that the game has been out for nearly 3 weeks, it has sold extremely well, and the reviews are glowing, there are going to be a lot of people who were borderline, who wanted to give the game a shot but were unsure, now they don't have to fork over $60 for something that they may or may not enjoy. The demo + all the word of mouth about the game post-release will give them a much better perspective and idea than they would have had if the demo came out pre-release.
Why not release the demo along with the game in first place?
Seems a ploy, if the reception was underwhelming they wouldn't have released any demo. The reception was great, so they are releasing it now