Kasich has been campaigning on a shoestring. Bush raised well over a hundred million, between direct donations and dark money to his "leadership PAC" which is prohibited from quote coordinating unquote things with his campaign. He burned through about a third of that in New Hampshire and came in at a level that came nowhere near matching the money that was spent.
My suspicion is that the donation river has dried up for Bush. He needed to do well in South Carolina to persuade the oligarchs to re-open the taps, but he hasn't. One has to wonder how long he can keep going before his coffers run dry.
Update: No further than tonight:
Former Florida governor Jeb Bush has dropped out of the Republican race for US president after failing to rank high in the first three state primaries. ... "Today I am suspending my campaign," an emotional Mr Bush told his supporters.Somewhere, the shade of Terri Schiavo is laughing.