Nobody thinks he's a locker room cancer. The reasons are:
1) In-season cap flexibility matters, we don't know what will happen and there's always a possibility that a team like Nashville decides to blow it up and Forsberg or Ekholm becomes available, maybe Lafreniere has a breakout year and the Rangers decide to move Kreider with Panarin and Lafreniere locked in to their top 6, maybe Hampus Lindholm becomes available in season and is willing to extend with us, there are a lot of ways we could allocate 5.5M (and 10+ pro-rated if we wait half the season) that are more valuable to the team than Drouin, who is at the absolute most optimistic view the 4th best winger on the team if you're low on Hoffman and Anderson.
2) He's just not very good, and the same roster congestion on the wings will exist next year unless another better player is traded. He's still under contract for next year, and right now is as good a time as any to fix that roster congestion issue, especially when we have Toffoli, Caufield, Gallagher, Hoffman, Anderson, Armia, and Lehkonen who are all very easy to pencil in to a top 9.
3) He's a bad fit on any of our top 9 lines. He is too weak defensively to play with Suzuki unless you're taking Caufield off his line which is also a bad thing to do, and doesn't fit very well on line 2 or 3 if they're centered by Dvorak and Evans and expected to take defensive matchups, and is likely not going to produce much on the 4th line with Evans/Paquette/Perreault + Armia, and we have internal options who will be basically just as effective in a 3rd/4th line role, especially if the 3rd line is expected to take some defensive matchups.
You can disagree with the reasoning if you want, and I don't think it's wrong to say you'd prefer Drouin at 5.5 over the empty cap space in the 2nd week of September, but it's dishonest to act like the only reason people don't want Drouin around anymore is some emotional vendetta against the player. We have 6 top 9 winger slots, and I think most fans believe Caufield, Gallagher, Toffoli, Hoffman, and Anderson deserve 5 of them. That leaves Drouin, Lehkonen, and Armia for the final 3, and given the likely makeup of our 2nd and 3rd line where Dvorak and either Evans or Poehling are taking defensive shifts, it's perfectly reasonable to prefer Lehkonen or Armia in those spots and question if it makes sense to use Drouin on the 4th line.