Why do you think speedrunning is so popular in Diablo 2 but never caught on in World of Warcraft. There are streamers that talk about different games and when it comes to Blizzard titles, Diablo 2 is far and away the game that gets the most speedrun love. There are 45 players streaming Diablo 2 (vs 25 for Diablo 3 and World of Warcraft). When streamers talk about which games they pick, they say that, overwhelmingly, streaming D2 speedruns nets them big crowds and subscribers. I admit I follow one guy (mrllamasc). He's pretty entertaining and streams most nights.
People who are a fan of LEVELING in the RPG gravitate towards Diablo 2 over D3 or WoW. Its pretty clear. Some people just love leveling chars as THE endgame and D2 stands the test of time.
Is it the RNG factor? There are several KEY points in leveling in D2 that can boost your time, from acquiring fast run/walk boots, to the stealth runeword, to picking the correct path in Arcane Sanctuary, to finding a great unique or rare that changes your build. WoW is more streamlined. The rewards you get are generally more on rails with little to no randomness. The map is always the same in WoW as well.
Torchlight 2 is another interesting question tho. Torchlight 2 has random maps and even random custom maps as endgame with random bonuses, but hardly ever gets speedrunners.
Getting back to WoW, I would think WoW speedrun would be popular because there are so many classes and you can take different builds. Enhance shaman vs Ele shaman for example. What could they do to increase interest in speedrunning WoW? Allow players to pick which starting zone to begin in? A Draenei Shaman in the human starting zone?