Boris's supporter MPs were on TV. They had no clue that Boris would not stand. Kwasi Kwateng was gutted and has switched his support to Theresa May.
Boris is probably gutted, but he understands how it works. If the real power doesn't back you and cuts the puppet strings, then you have to suck it up and back down. Boris will get over it, he has read enough Julius Caesar etc to know how it really works.
A lot of these MPs don't understand how it really works, they are blaming Gove, but the real power, represented by Dacre and Murdoch, urged Gove to stand because they don't trust Boris. If Boris thought he stood a chance, he would not have withdrawn. Boris knows that the power does not back him so he had no option but to withdraw rather than stand and lose like Liam Fox or Andrea Leadsom etc