1. PvP gets the shaft in favor of CooP PvE.
2. PvP and PvE exist in the same "zone". Basically Open world PvP. This works better for MMOs as people go to an MMO not only for massive multiplayer gaming, but to experience another world. Structured PvP can't do this. Things like WvW or a full PvP game like EVE achieve this - because your actions affect the world and aren't just a one-off encounter with leaderboards.
Granted this is personal preference, but I think when it comes to MMO games, a large majority of players don't want the GW1 sPvP model. Not because it is bad, but because the market is saturated with games like that already. I already play COD, League of Legends, DOTA2, Battlefield for the structured matches with leaderboards, K/D ratio (doubt GW2 has this), etc. I don't want ANOTHER game like that, even if the gameplay is a bit different. I believe this is how a lot of players feel, they buy GW2, or another MMO, for the world not the PvP. Simply because sPvP Match style Multiplayer is everywhere and a lot of people don't want to play the SAME gametype over and over just with different mechanics.
And yet, GW1 did it really well. Many mane people would suggest that GW1's PvP was the best MMO PvP ever made. And the PvP in GW1 was basically a MOBA.
Yes, they ran in to problems with balance when they introduced new professions and skills - but those things could have been easily remedied in GW2 (and many thought that was part of the reason for making GW2).
So, to say that it couldn't have been done seems a bit silly considering that the same company that made GW1 made GW2 and GW1 did it better than anyone.
Furthermore, the state of PvP in GW2 is so bad that, comparatively speaking, it is a colossal failure.
Also, none of the MOBAs you referenced hold a candle to GW1s PvP, I merely mentioned LoL because I anticipated the common argument that PvP doesn't matter because it isn't a money maker.