He was a problem for team harmony and Simeone wanted rid. It's not as if the sale was a closed auction and Carrasco had the choice to say no at any time. The fact is Atletico sold to the highest bidder and the main dodgy thing was the underhand payment to Atletico as the inflated amount was an artificial boost to their coffers (re: FFP).
Spats with Koke, Gabi, 'Profe Ortega'
Carrasco has proved a problematic figure in the Atlético squad on several occasions, with the Belgian involved in bust-ups with team-mates Koke and Gabi, and fitness coach Óscar 'Profe' Ortega, in the past year alone.
And despite Carrasco's ability to bring a level of flair to the attack that is offered by few others in the Atleti first team, Simeone is said to be ready to admit defeat in his attempts to tame the 24-year-old.
https://en.as.com/en/2017/12/27/football/1514364857_611109.html