Microsoft has invested a lot of money into the future of this franchise, and I think they're gonna hype the hell out of it to make it pay off. Obviously Halo games sell well, but the series has a lot less momentum than the Halo 2 and Halo 3 days. The rampant popularity of Call of Duty is an undeniable part of that, and I think MS is gonna try hard to win the Halo playerbase back.
And after the lukewarm reception of Reach, I think a lot of people need to be convinced about Halo 4. The mystery of a new world/new trilogy is a draw for sure, but for a limited number of people. The fact that it's a new dev handling the franchise creates a lot of doubt in people's minds.
Yeah, there's really just too much uncertainty amongst gamers. MS and 343 need to counter that, and build hype big time. I think a beta could be a great way to build momentum.