Chief, competitive e-sport players have, for the most part, very similar skill sets; taking the direct example, SWTOR is mechanically very similar to WoW*, and as such, one might expect a top level WoW player to have a pretty solid handle on SWTOR.
Furthermore, the key to competitive success is far more often the players mindset, and not the use of actual mechanics. When you look at a players success in one game and take the actual mechanics out of the picture and focus on mindset, you're left with the conclusion that said player has a universal leg-up on 'regular' players in any similar game that player plays - like going to SWTOR from WoW.
*WoW is not technically an e-sport, but neither is SWTOR. The argument should apply the same regardless.