He only did it because Dana gave him a big-money deal and agreed to put him in prominent PPV spots despite having no fighting background. If that hadn't been the case, Punk wouldn't have done it.
If he had guts, and if fighting was something he wanted to do, he would have invested his own money into it and tried to prove himself in amateur/low-ranked competition first.
What he did was use his status, built in the WWE, to money grab and play pretend. It was his dream, his fantasy, to be a UFC fighter, and he got that, not by working hard for it. No, he did it by leveraging fame he acquired from an organization he vindictively buried and all he could muster were two hilariously awful, meek performances.
He disgraced himself and all of wrestling for his own ego. Few other wrestlers would be able to get the deal he did in the UFC and so rightly wouldn't bother, but even among those who could, I doubt they'd want to do it, because they would have some semblance of humility, shame, and a recognition that as a pro wrestler with no professional background, it is disrespectful to both the sport and its real athletes to come in, take their money and time, and then make a joke of it.
