343i's plan from the beginning has been to test the player populations and demand for certain playlists by adding weekly rotations (which will resume after the Infinity Challange ends) and taking out playlists with low populations. This achieves two things: It shows what playlists become popular when first entered (SWAT, for example), and which ones don't do so well with the numbers (Team Snipers) before throwing in everything at once. It also increases player population in the existing playlists so the numbers aren't spread so thin over too many playlists. I'm sure once they find where the demand is for playlists and make necessary adjustments (like they're doing to Team Snipers) they will add in some more permanent playlists. Plus, a FFA playlist will be coming later in February.
As for the missing gametypes, it could be a number of things. Personally, I think it was a mixture of both creative choices and time restrictions. Assault being taken out seems like a creative choice, since it was one of the weaker gametypes as far as numbers were concerned, and there are many other competing objective gametypes. As for One Flag CTF and Race not being added, I'm guessing those were due to time constraints. 343 had a LOT to finish to meet their deadline of a November 6th release date. Perhaps they didn't have the time to add in a few more gametypes and instead had to focus their efforts on polishing what was there as much as they could. The same thing happened with Halo 2. Time was short, and necessary cuts had to be made.
Again, this is just my opinion, but Franky has even said that they realize the game isn't perfect and will do better next time. It's their first game, and they had to build everything from the ground up and didn't have a lot to jump off of with the divided fanbase that came with Reach. Cut them some slack and look forward to the future. I know I am!