Not surprising at all. In fact,
I predicted it.
He put all his money and effort into Iowa thinking that a win there would propel his campaign forward to New Hampshire, South Carolina, etc. etc. He vastly underperformed
his expectations in Iowa (though not pollsters), and was already bleeding money and losing donor support before the caucus. Without a win, no money, and no ground game in NH or SC; he had no path forward.
The upside to this is that we now have a two horse race. Most of the DeSantis supporters are going to vote for Trump, because that's who he's now endorsed, and those are the voters who he was trying to appeal to. But now there is a clear delineation, Trump or Haley. This hopefully will make things a little more competitive and interesting.