Because he was bought for enormous money which put a lot of expectations on his shoulder and There was never a plan for him. He played best centrally as a second striker or striker But we already had suarez There and we had messi as a right winger so we couldnt even use him on his third best position. We ended up using him as a left winger were he sucks and had No possibility to invert Too because of him being left footed. So basically we limited all his greatest strengths, he didnt have Good confidence and all of our balls were going to Messi so he didnt have the chance to show himself a lot.

It got Better the second year after suarez left But still not close to his atletico years and after that second year we ended up selling him for a bargain price because of his rididiculous salary (given by our infamous idiot president at that time But thats a different story)

Messi occupies the same spaces he drifts into so he was forced to play on the left