Scale Shot is a tricky case, I think it's excluded because, as
R_N said, it's the only move which has a batch of traits to it: It's a damaging move, a multi-attack move, a self-buff move, and a self-debuff move. It's a move wearing a lot of hats GF considered balanced but now we're talking about removing a block in this Jenga puzzle.
Hypothetically, going by the rules of other moves, had Scale Shot been affected by Sheer Force it would become a multi-move with a Power of 32.5 but decreases the user's Defense stat each time it hits (Sheer Force would have removed the Speed increasing attribute). That would make it the highest Power multi-hit move, since it strikes 2-5 times it would have the effective Power of:
2 Hits/35%: 65 (normally 50)
3 Hits/35%: 97.5 (normally 75)
4 Hits/15%: 130 (normally 100)
5 Hits/15%: 162.5 (normally 125)
Weighted Average: 100.75 (normally 77.5)
Note the only other multi-hit move affected by Sheer Force is Twineelde (because it Poisons, all others either have no affect or their affect is more of a passive trait) which only hits two times with a Power of 25, so it's not doing much damage (if anything removing the chance to Poison removes what little thing makes this move unique).
Now, many would argue the Defense increase without added Speed should be enough of a counter to the increased damage output, but I think that one is besides the point. As the saying goes, the best defense is a good offense. You give this move to a high Attack dragon that's either already fast carrying a Power increasing item or one with decent Speed carrying a Speed increasing item and you now have a dragon that 90% of the time hitting the opponent with a Move that is as powerful as a Z-Move/Max Move each turn. Only thing that would stand a chance would be Fairy-types but that's because they're immune. Also, being a multi-hit move, it gets a few additional benefits:
- Each shot has its own chance of being a Critical Hit adding a little more Power for each one.
- It "gets around" Substitute, Focus Sash, Focus Band, & Sturdy.
It was likely Scale Shot was skipped over because it was considered just TOO powerful when the numbers were ran. Sure, the user would be a sitting duck (or rather a sitting drake), but after they get to -6 Defense they got nothing else to lose and if they can hit their opponent hard enough to OHKO that's not gonna be a problem is it?
BTW, this is pretty much a "future proofing" decision. I think the only Sheer Force user that can learn Scale Shot is Druddigon and even if Scale Shot was affected I don't think Druddigon would be a force to deal with (maybe on a Trick Room team, but otherwise). Moving forward I think we can add multi-hit moves akin to Scale Shot (can hit up to 5 times) will not get a Sheer Force boost. Now, as we've seen with Twineedle, maybe a 2-hit and maybe even a 3-hit Move could still, but that remains to be seen.