Niasse was repeatedly told by Koeman during the summer transfer window that he has no future at the club, but stayed at Goodison Park nonetheless.
Koeman admitted that he "can't understand" the player's decision to stick around, and stressed that his goal during a recent Under-23 clash with Arsenal will not improve his first-team chances.
"I don't think the goal changes anything but he's still a contracted player of Everton. Of course I mentioned before the season started that he's a player who will not be in my plans but he's still under contract and he needs game-time for if there is interest from other clubs," he told reporters.
"Normally, we have enough strikers and different strikers to what he is. I prefer these people and not Niasse in the team. [Why he stayed] is a question for the player. There was some interest in him but not the interest that he liked to have.
"It is all up to the player whether he doesn't leave. I can't understand it but that is normal. Football players like to play football, that's how I see it, and I explained to him why he was not in my plans for the season."
Niasse has made just two starts for Everton since joining the club for a reported £13.5m in January.