Because it does not make sense to keep Mahrez.
We have Sane left who had zero game time last season.
Instead of him we can have Messi+Torres. They both want game time and Messi is not the type to sit on bench. Not in PL and CL. He wont sit on the bench at all if he is not injured.
So who will play less in the wing? sterling or Mahrez?
Is Mahrez ok with 5 PL starts and many mor as subs and couple cup games?
Does it worth it to keep him for that when we can still sell him and Torres could take his game time?
Bit like the issue of Tevez, Kun, Dzeko, Balotelli, great options but never really worked outo keep all of them happy.
It was an overkill which was very hard to maintain.
Same with Mahrez, Sterling, Kun, Jesus, Sane, Bernardo, Silva, Foden, for about 4 positions. Sane being out gave everyone else more games but that was a very unlucky injury for Sane.
Both Bernardo and Foden wanted to play more which they can now with Silva gone.
Silva started 22 PL games last time, Bernardo 23, Foden 9 but Bernardo sometimes started on wing.
Mahrez started 21 and thats with a great winger option Sane being out.
Imagine if Sane is fully fit, Mahrez wouldnt started 21 but maybe only 10.
His debut season he only started 14.
Now add Messi and Torres to that.
No way Mahrez would start 10 PL games in that case. Just no way but I would rather give that game time to Torres who could be the future and Messi would play as much as he can if he comes because wont be benched him everyone knows that.
He is either starting or out injured, Messi wont wait for Pep to make late subs in 80 minute to send him on come on...