Because Rennes are absolutely stacked for attacking talent and he is only 20, so in those situations, younger players tend to be benched.
Doku is 20 too but was there before him. They also have Terrier, Kalimuendo, Gouiri and (even though he's not a forward) Bourigeaud who plays on the right side of midfield. Probably a few more too whose names escape me. Sulemana is a young lad who will want to be playing regularly.
Hello, I am a Rennes supporter and regarding Sulemana's playing time, I can give you some details. First, the context: Rennes has just lost until the end of the season its best striker, Martin Terrier, and since Sunday, another titular striker, Kalimuendo. You might think it's a chance for Sulemana, but he doesn't seem to be able to take it. It should be noted that Doku, the other young striker, has also played little because he gets injured very often. Sulemana had two or three good games when he arrived last season, then disappeared, either injured or a substitute. His coming into play was not convincing, he has a lot of trouble adapting to Rennes' collective game, his dribbling no longer works, he sometimes lacks game intelligence (losing the ball against Fenerbahce leads to the equalizer Turks in the Europa League), and despite the patience and advice of coach Bruno Génésio, he is not progressing. Now, in another context, another system, maybe he can better use his interviewing, dribbling and speed skills. Good luck to you.