Well, the problem is Trump is in a bit of damned-if-you-do, damned-if-you-don't situation. If he gives it to Romney, he's opening the door to a substantial amount of criticism, not unwarranted, since after all, Romney did commit himself to stopping Trump. It does look weak, and it also sends the wrong signal about how loyalty is valued. Many Trumpists think that Romney should not be rewarded, and in fact, should be punished by being ostracized from the GOP, and they have a point.
On the other hand, Trump is limited in his options. Trump's foreign policy is basically the Polk-esque, Jacksonian model (i.e., generally somewhat isolationist, while still engaging in necessary realpolitik, and avoiding conflicts unless it means we're going all-out, scorched earth style). Unfortunately, this model hasn't been in vogue for over 30 years, arguably since WWII or even earlier, so there isn't exactly a deep bench right now. It's basically either Romney, or "insert generic neo-conservative," and neo-conservatism is really at odds with the Jacksonian style model of Trump's. Trump probably figures that Romney, though he has some neo-con tendencies as well, is as good an option as he's gonna get, and at least he's competent.
The ideal match for Trump would be someone that has a mix of Rand Paul's reluctance to get involved with foreign entanglements, with Bolton's all-out aggression (when we absolutely have to get involved), but that person, sadly, does not exist.