I think they've said the next debate will be limited to the top six, based on polling in New Hampshire. Which got a big row out of some people, because you could potentially have Carson with a 4th-place finish in Iowa then being excluded from the main debate, while you'd have Kasich onstage, who'll be lucky if he gets 6th place in Iowa (currently polling around 2% there). ABC News is also planning to NOT have an undercard debate, which would exclude Rand Paul and the usual zoo crew we've gotten used to at the kids' table.
EDIT: One theory that I've seen is that because Santorum was polling in single digits up until about a week from the 2012 Iowa caucus, it's given false hope to all the single-digit candidates. He was only polling around 15% in the last polls prior to the voting and wound up winning with 24.56%). They're all hoping for the same Iowa miracle, but the thing is -- Santorum had basically lived in Iowa since the start of the race. He poured literally all his campaign into Iowa, and drove around to all 99 counties. None of the candidates this time around has done that, to my knowledge. Santorum put all his money on 17 Red and got lucky, but it wasn't enough to last him. Everybody this time around has been trying to play Iowa and NH at the same time (or focused on playing just NH) and often several other states like Florida, South Carolina and Nevada. Not only does that take a lot more money, it cuts down on the penetration factor.