For one thing, Nick and Sally barely interact unless the MC very specifically wants them to, which means no one can really see how they are together. Consciously or not, this may be to prevent the reader from feeling “forced” to ship it, which is fair, I suppose, but which might be contributing to lack of Snickly fans. That, and the ship name - it reminds me of a whiny mustache-twirling villain draped in black. It also sounds rather similar to “sickly”.
Ship names and non-existent interaction aside, I’m not a fan of what’s currently in the game regarding the ship. Currently Sally comes off as kind of borderline obsessive over Nick, rather than just caring for his wellbeing. She also has kind of a strange attitude of entitlement to him, which comes through especially in the hospital scene and really turned me off from ever shipping them, though I suppose my interest in a Sally-romancing playthrough might be leaking in there. Still, that was the moment where I sort of made up my mind: no, I’m not setting them up, not even on my Gray playthrough.
In addition, the hospital scene also has the effect of unintentionally showing “Sally cares more about Nick/her feelings for him (more the latter in my opinion, tbh) than the MC” because she wants to go in before them, which really does come off quite badly, and makes me, and I’m sure many other who wanted a close friendship/romance with Sally (or close relationship with Nick), resentful of the route
Their personalities don’t seem to work very well together either, in my opinion, as has already been said.
That’s most of it, I think. Though I see now (upon reading it) that many of my points have previously been touched on by the post above me, I hope it’s still helpful anyhow.
EDIT: Forgot to add that her being a RO makes some difference to me, but I’d feel at least half the things about the pairing I do now even if she weren’t.