Slash itself is wildly more popular than heterosexual pairings, that wrestling boils down to good-looking, half-naked men rolling around while often times covered in sweat does not help, with all the Ho Yay and Foe Yay abound outside of the actual matches.
The majority of heterosexual pairings in the fandom are not the male & female wrestlers in regulation pairings, but rather the Male wrestlers with female Original Characters. There are vast numbers of poor quality Self-Insert Fics.
AJ Lee was in an Official Couple or had Ship Tease with a grand total of six guys... in one year. This was bound to happen eventually. Of those six guys (Daniel Bryan, CM Punk, Kane, John Cena, Dolph Ziggler, and Big E. Langston), her most popular pairing is with Punk, despite their interaction ending very quickly, becoming out of character alignment and AF ending up with Ziggler in canon for well over a year after Punk.
The Shield. No matter how popular other couples comprising of them are, it can't compare to how much the fans like to ship them with each other, especially after they broke up. The most popular, of course, is Dean Ambrose and Seth Rollins (Ambrollins), though adding Roman Reigns to the mix and making it a One True Threesome is certainly not uncommon either.
On the heterosexual side of things, Seth is most commonly paired with Paige (most likely due to their similar tastes and looks and the fact that they were supposed to have a romantic storyline together down in developmental), while Dean's Real Life relationship with Renee Young has been happily embraced by fans.