I believe a few MMA guys have had some pro boxing bouts for sure, but I don't think there has been a high profile crossover on either side really.

It stands to reason that a boxer has a better chance of crossing over to MMA than the other way around. Boxing is, of course, a martial art. The vast majority of guys in the UFC at least are only truly proficient in one or two individual forms of combat. It is nearly impossible to be dominant in several at the same time, which is one of the reasons it is so rare for guys to go undefeated for a length of time in MMA, unless they are simply far more athletic than most anyone else around their weight divison(thinking Anderson Silva, St Pierre etc.)

A serious MMA fighter will have no less chance to be succesful in boxing than he would any other individual martial art, which is still next to none at the elite level. How could you possibly beat somebody who has trained his whole life to fight under a certain set of rules, when you have to be prepared for all kinds of different things from one fight to the next? What you are asking is no different than "why haven't more Triathletes crossed over to competitive swimming?"