Gazzetta dello Sport tried to answer to this question.
When Perisic asked to leave, Keita had his chance to impress. And did impress. When Icardi ruled himself out, Keita was the only ‘made’ player that could take place in the striker position. But he lost himself.
Apparently, the injury that stopped him in January was aggravated by a practical joke that a TV show did to him, with his girlfriend as accomplice, just before the Sampdoria game on the 17th of February. Footage of the joke was never aired because Keita has rejected to sign a release.
Keita then returned on the field only in the middle of March, but Icardi and Perisic were back in the game plan.