Ferban up there already answered some of that, but I'll add that only registered users can leave a ranking (and only one obviously) and it takes a lot of vote to influence the ranking. For example for Underworlds, there are some 2800 users that rated the game.
There is certainly a bit of that, but many other GW games did very well and let's not forget there are more than 125 000 ranked games on BBG, so it is not the worst thing being at 4270.
BGG's ranking reflects the opinion of the whole gaming community, not just the wargaming one, so as much as everyone here might like the game, we would be kidding ourselves if we said AOS has the possibility to become a fan favorite game across gamers from all around the World.
This is true, but aren't wargames just a subtype of board games? It's been a long time since the expression referred only to traditional games played on an actual board.
Sure, but again, the list I gave prove some of GW games are considered excellent as well.