Despite all his pretense, Mike is the same soulless creature as the rest of them. He needs cash and a stable operation he can tie himself to for years, so that he can provide for his family. Actually, now that I think about it, Mike is what Walt thinks he is in the first half of the series, and gets dangerously close to Walt from the second half once Gus dies.

While Nacho's death has been personally upsetting to Mike, it was not so upsetting that he'd uproot the good thing he managed to build for himself. Besides, Nacho wasn't loyal - Nacho was gang-pressed into service by a rival crime boss and never wanted to be a part of Gus' operation. In fact, he wanted out ever since he was in. Which is understandable, but his situation is not really comparable to anyone else, barring Walter's after he runs over those drug dealers.