I'll just say because it's not going on sale because it sucks.
I'd say the real reason it is not going on sale is that generally when you have a game like that, people will pay whatever for it. In a way it is better for them to not discount it.
Example:
I have $60. I can buy Halo 4, because I enjoyed the previous 3 games, or I can buy 3 $20 games, like Batman, CoD, whatever. However, because a lot of people like Halo, they will buy it anyways, possibly not buying anything else, so they are going to play more Halo which means less money and time for other games. I'm not sure how the community is, but I would venture to say there are less people playing every night after 6 months than after 1 month.
They'll make money and keep people from playing other stuff until the initial thrill of the game goes away, by that time they mark it down and don't lose much, since they already made a ton earlier by not discounting.
Nintendo seems to do the same thing with their main franchises--Zelda, Mario, Mario Kart, Pokemon. Pokemon Black still costs $30 or $35, and the sequel is out for $35, but people buy them both!