The main reason pertains to the degree of unfairness. In Standard RSB two males kick each other in the nuts until there is only one left standing and he is the winner.
However, if one man has stronger legs than the other man, the man with stronger legs is given the advantage. Up to date this method is largely becoming illegalized and has been upgraded for the new and improved "Professional Ro Sham Bo".
(Although "Standard RSB may still sometimes be played for fun.)
In Professional RSB there is normally money or spoils at stake. Therefore Professional RSB is often mandated. Also any opponent may at anytime call out for Professional RSB and if so, must be enacted.
Rule #1) In Professional RSB also known (as Third Party Ro Sham Bo), one or more unbiased kickers must be nominated. If a biased kicker is nominated towards one of the opponents than yet another kicker must be added of equal bias to the other opponent.
The kicker must kick each opponent than step back and let the other kicker proceed for their round. And on and on in this fashion.
Rule#2) BOTH opponents must wear speedos. This is to ensure to the video taped audience/judges/spectators that there is no wearing of "cups".
Rule#3) An unbiased referee must be present to nominate kicker/kickers who must be of equal bias towards both opponents. The ref will call out the nature of the spoils, (normally money from the spectators but not always)
Rule#4) GIRLIE KICKS ARE STRICTLY PROHIBITED!
"Fuck you! I'll play you but Dean's not going to be Ref, anyone but him, I don't trust you two. And we're playing Professional Ro Sham Bo or I'm out."
"Your on! Jenny's Ref then."
"Fine, Jenny, who are you nominating as Kicker?"
"I am nominating BOTH Lisa and Carrie as kickers."