Because the games he played well were few and far between. Fewer and further between than Suzuki's. Are you completely forgetting he was sent to the A last year?
That's not even true...and yes, I remember he was sent to the AHL last year.
Kotkaniemi was having a solid rookie season, he was trending towards a 40pt rookie season until before the trade deadline, CJ decided to remind everyone of his age and that he was tired, so they brought in Nate Thompson, who just like Staal, took some of the few responsibilities and opporutnities Kotkaniemi had and Kotkaniemi's confidence was wiped to end the year.
His second season, the Habs re-sign Thompson...so what do you think happens? He struggles playing a bit role...eventually he gets sent down to the AHL (not defending him here either, he didn't look good).
He comes back for the playoff bubble...everyone thinks he's going to sit, but because Domi has health issues...Kotkaniemi suprisingly gets to reclaim that 4th line center spot, with Thompson gone he doesn't have to worry about him. He gets off to a hot start in the playoffs, to the point where by game 3, he's actually relegated Danault to a 3rd line role (which causes Danault to publicly complain). Kotkaniemi responds by being one of the Habs best players during the playoffs, he has a few games where his icetime actually exceeds 19 mins. He's actually on the ice in some moments when the Habs need an extra attacker. He's playing with high confidence.
Flash forward to his year...he starts out in his usual 3rd line C role. Danault struggles, Kotkaniemi gets off to a decent start...6pts in 10 games. As the season continues, Kotkaniemi continues to play solidly while Danault eventually finds his groove. The Habs eventually trade for Eric Stall, replicating the Nate Thompson sceneario from 3yrs prior, and unsurprisingly...Kotkaniemi's confidence goes down the drain.
That's pretty much a resume of his time with the Habs.
You want to knock him for not being consistent...OK, fair.
But the coaching staff and management decisions haven't done him any favors either.