Let's go point by point.
1. Scizor is just on 1 out of 4 teams.
2. Stealth Rock is just on 9,1 of 10 teams (at least on the previous generation).
Ok, see the difference? When something pass of the 25% of the entire game enter in what I call excesive omnipresence. Why people use too much Stealth Rock? Because, under normal or any logical analysis, it is OP as hell as it will make more than 300% of damage per game on a single turn (considering the standar number of switches). Yes, I know that there exist Defog and Rapid Spin, and so what? Think in that way: The spinner needs to waste a turn using those moves, the same move number you use on setting it, so, in the worst scenario, you're just not losing nothing. Well, Defog is a that good move against Stealth Rock? Not too much as most of its users are weak to it, yes, the same reason why Ninetales had more problems against Politoed on the previous generation, so, it will not be a too usual move, and those who use it will still be using Stealth Rock for its OP status anyway (the flexibity of SR + Defog is far superior to the one of only Defog).
Ok, now we know Stealth Rock is omnipresent, and it is OP. Well, what does it make to the metagame?
1. Any Pokémon weak to it will inmediatly fall 2-3 tiers, without questions, UNLESS it does have a way to surpass losing the 25% or 50% of its life, in other words: Cloyster, Gyarados, Ninetales, Thundurus-T, Dragonite, Salamance, Volcarona, end list. What is common on all of them? Easy, all of them have an ability to exploit an enter or defense (Intimidate, Roost, Multiscale), an OP ability (Ninetales) or just incredible high abilities to sweep in few turns.
2. Let's see how many types are weak to rock... 4 of 19, 21% of the entire metagame on every tier is almost completely impossible to use because Stealth Rock.
3. Switching is not more an option at all with few exceptions, with that you reduce the number of possible actions from 4 + 5 to just reducing strategy oriented options = you reduce the need of skill and increase the need of luck. Luck is less competitive, less competitive is clearly not good for a competitive metagame.
4. Because SR, too much other things, specially the obligatory use of spinners for a lot of teams, reducing even more the viable ammount of strategies and thus, the number of strategies to study, and thus, even less study and skill is required to win.
Reasons to ban something:
1. It is excesive omnipresent and focus the game on it.
2. It is excesive overpowered, with a power that never have penalties and always have huge rewards. (You have absolutely no reason or almost not to not use it if you want to increase your chance to win).
3. It reduce the need of skill and increase the need of luck. (Shutting down more than the half of the options on a battle and limiting an excesive numbers of Pokémon to very few).
Well, SR fits on everything on any normal criteria to ban something, curious, nope? Well, that is. That SR is needed because Volcarona and Dragonite will destory everything? WHAT stop us to ban Volcarona and Dragonite if those theoric (specially considering that removing SR will change the ENTIRE metagame) scenarios comes to be true? Nothing? Excelent, let's proceed.
The unique reason to dont ban Stealth Rock and why it wasn't banned earlier is because tradition. You're used to it? It makes you use lees the brain and makes things easier? Well, people will always lobby for those things, but that ins't competitive nor rational (and broke our current and any suggested set of rules to ban things), mind that.