There are few people more hot right now than J. J. Abrams, and NBC figures that he could be their best bet to provide a hit that can compete against the other networks. And boy do they need it. Heroes, as great of a show as it is, is still losing viewers and probably won't see a fifth season.
There's no way they can keep paying what they are paying for that show with five million viewers. And when Heroes goes, so does the only show that people really like on that entire network that isn't "reality".
Ironically, Abrams most recent show -- Fringe on FOX -- is having delayed ratings problems of its own. It seemed to have settled around 8-9 million viewers last season which isn't all that bad if they could have sustained it, but FOX for whatever reason shuffled their schedule around like they do every single year, and Fringe is paying the price for that.
In fact, Fringe is getting pretty close to Heroes and recently canceled Terminator: SCC territory; about 5.7 million overall viewers is losing almost 37% of its lead-in audience from Bones, which is still doing very well. Fringe averaged 9.9 million viewers last season, while Heroes averaged 9.2. Ouch.
THR says that a new series spec (a series pilot script written on the speculation that it might be bought) sparked a bidding war between three of the big four networks, and that NBC came out on top. The series premise is a drama about a husband and wife team of spies. I don't personally like some of Abrams stuff and I think he's probably somewhat overrated (but still very talented), but it doesn't take a genius to know that NBC needs this badly, and it'll be a big step up for them in quality from some of the crap they've been putting on the air this decade.