We are in a liminal state right now where it's unclear what rules apply, where the state of play really is, and so on. Trump's second-place in Iowa was barely within the polling margin of error (Cruz at the top of his, Trump at the bottom or maybe just below it--and there is at least some evidence that the polls were very fluid going into the caucus).
So we don't know if the NH polling is accurate or not. We don't know if events (tonight's debate, etc.) will change things. We just don't know.
Secondly, Cruz won by having a great ground-game which is amplified in caucus states (see: Obama '08). He probably has a great ground game in NH--but he may not get the same kind of boost for it. Trump shows as WAY ahead--if that's for real--if people turn out--then he wins NH. If he wins NH credibly, there is no reason to think he won't win the other early-states.
Rubio has certainly locked up the branding as the establishment / unity candidate--the question, which has ever-dogged him--is can he close the deal? What does he need to close it? Will a convincing 2nd place be enough? Will a squeaker-second? What if he comes in 3rd?
Similarly, Bernie is way ahead of Hillary in NH. Assuming he wins it, what then? No one knows--the demographics show that Iowa and NH are Sander's best bets--but a recent national poll showed him tied with Clinton. Is he catching fire? Is Hillary losing another national race?
The underlying fundamentals of the races haven't changed: Sanders would likely be a disaster for the Democrats and however shaky Hillary sometimes appears (she's pretty solid in debates--but she's dogged by scandals and lack-of-transparency issues) there just isn't another candidate (yes, there are Draft Biden fantasies--engage them after Hillary goes to jail--not before).
The Problem: Lack of Real Options
On the GOP side, Trump and Cruz are still seen as un-serious and potentially catastrophic choices. Trump because he is a cartoon supervillain (and has the favorables to show for it) and Cruz because it is apparently he makes being hateable into an art-form (also because he appeals very, very strongly to narrow groups of people who may well not be able to dominate swing states in the fashion necessary to win).
Additionally, every other conceivable establishmentarian has more or less flamed out. Oh, sure, there are trace elements of Kasich and Christie in the polling--and Jeb's superPAC has a lot of dough left--but mostly? Rubio is the only guy who seems like a credible choice right now.
Essentially Tuesday's coming vote is forcing a lot of pressure into a system and seeing what, if anything breaks.