Blame old and naive Wenger, not Alexis. He has been their best player this season.
Ok. I am not saying that this is the case with Alexis, but in a history of football and in a history of other team sports, there were players:
1. who had good individual stats (Goals, Assists etc)
2. but the team in which they were playing, just always performed averagely, despite Goals/assists/good individual stats from a player.
Example in numbers, imagine a team scoring 60 Goals in one Season, and a player Xx scores 20 Goals.
But the whole team just won less points than expected.
And then, they sell that star player who scored 20 Goals, and they buy a weaker player on paper who will score only 10 Goals per Season...
= but then in the next Season, without that key player (with 20 Goals) from the above, suddenly the whole team plays differently, and suddenly they score 80 Goals, +20 goals more than with that key player in the team
-- without him, the hole team plays differently, and they spread the actions and number of shots differently
Again, I am not saying that this IS the case with Alexis, but after his Barca and Arsenal spell, one of these two scenarios is probably correct:
1. he is an awesome player. He has good individual stats, but he was unlucky both in Barcelona and Arsenal, that the rest of his teammates had poor Season in those Seasons when he was in the team, so his individual brilliance wasn't enough for (major) trophies.
2. he is a type of player who has some awesome skills, and he has good individual stats, but apart from individual stats, his presence just don't bring anything special to teams in which he is playing, in terms of much more points than usual, or in terms of trophies.
In shorter, a good player with good individual stats, but not good enough to turn team's results on longer terms (either with a help of other teammates, either as a one man show like Zlatan, Messi or Suarez, who can alone turn club's results).
And more or less, a team will have the same results with him or some other, random, decent player.