Just like Palin and Cain originally endorsed Newt, just like everyone who endorsed a GOP candidate before Romney did before Romney became the presumed nominee. This is so common that it's basically academic at this point, and whether or not you agree with that there's no doubt that it's done for a reason; party loyalty. It basically amounts to a loyalty pledge to the GOP to endorse the presumed nominee after you endorsed a losing opponent of his.
He endorsed Ron Paul and then stabbed him in the back. GTFO.