It's all about the ratings and ad money. Splitting the season across two years means they'll have Breaking Bad viewers for another year and they can keep the show's advertisers for another year, too.
I get that. What I'm wondering is why they're splitting season 5 in half. Why not just do a season 5 & 6 instead of season 5 part 1 and season 5 part 2? Either way, they're still getting the ratings and the ad money. The only reason that I can think of has to do with contracts needing renegotiated. They might not want to pay the cast/crew for an extra season and all that comes with it; more money per episode and stuff like that.
AMC has had a lot of contract negotiations go sour in the past, so perhaps this is just them trying to bypass some of that. Contract writers/actors/etc. for 5 seasons, but then make each season twice as long. You can then make each half of a single season into a mini season, so you're basically getting two seasons in one. They're doing that with The Walking Dead, both this season and next, so I wouldn't be surprised if it became a "thing" for AMC. (Mad Men, The Killing and Hell on Wheels aren't doing this in their upcoming seasons, to be fair)/speculation
The only other benefit that I can think of for splitting the season in two would be not having to wait a year in between episodes, but that doesn't seem to be the case here.
