You say Mount and Blade melee is easy? ANY half decent Native player would whoop your ass all day long. I'm sure most NW players would too, to be fair, but saying that Mount and Blade melee is easy is retarded. Sure, the basics of NW are easy, as you said, two directions. However, once you factor in chambering, holds, holding chambers, dolphin diving, kicks etc it becomes much more complex.
Going further than that, the whole point of NW is not a 1v1 duel. Duelling a player can take ages if they're roughly the same skill level, but the whole point of this game is the teamwork. Groupfighting is one example of that, but it's not about that either, it's simply one competitive scene that has developed to hone people's skills. Real skill comes from the communication, leadership and teamwork involved in a proper linebattle. Not a 1v1 lb, but a fully fledged linebattle with skirmishers, cavlary and artillery as well, all coordinated through teamspeak to destroy shit. THAT is where the game becomes so much more than other FPS games. That is where the game is at it's most enjoyable, but unfortunately most lbs are disorganised with regiments having to react to the movement of regiments on their team as well as the opposite team, with half of them ignoring the rest of their team.
Coordinating and planning 100 players of 7 regiments/groups and 4 classes of troops is difficult in and of itself, but it's where the real experience is. It factors in the grasp of tactics of the commander, the tactics of each leader in moving, positioning, engaging and making their own decisions, the teamwork of each regiment and the individual skills in melee of a player.
Sure, it might not be your cup of tea, but there's plenty of reasons that people enjoy playing it, and the myriad of competitive outlets that are on offer are part of the enjoyment of it.
Compare it to CoD/CS:S/BF/WoW/LoL/DotA2 etc, which all have competitive scenes, and I think you'll see that NW has at least the same level of competition, we simply don't have the player-base to support the same levels of competitions, and you aren't going to get enough players who are at the highest level to draw that big of a crowd.