Why? Because Smogon doesn't actually care if something is 'broken' or 'centralizing' gameplay, they care if the simple majority of their community doesn't like it. There's an argument here about counters to what gets banned, and how easily it can be dealt with. But when you compare the amount of moves, that ignore evasion boosts, hit for double damage on evasive targets, and the abilities that ignore evasion, and on and on about evasion - and compare THAT to how many ways you have to deal with stealth rock? It's a stark and harrowing contrast. Smogon LIKES rocks; but they don't LIKE evasion. Despite how many means there are to dealing with both. (Don't get me wrong I don't like evasion either; but this argument stands.)
Evasion is LUCK based. Luck doesnt add anything to the game and turns the game into a crap shoot. Huge difference. Also moves that hit evasion are a steaming stinking pile of grade A Montezuma monkey crap. Also if the community doesnt like something thats a pretty big indicative that its broken.
This applies directly to their mega bans too; most of those banned megas share a lot of common common weaknesses. Lucario, Kangaskhan, Mawile, NONE of them like a simple burn. Kangaskhan usually can't even hit ghosts with Power-Up Punch, Sucker Punch, Return and Fake out! (yes, I know sucker punch, but since that's its only option, and it relies on you attacking, in singles you can literally just sit there and use will o wisp repeatedly and it can't touch you.) Lucario, and Mawile, BOTH weak to ground, and fire. Very common offensive attack types.
Khanga gets Crunch js. Nothing outspeeds Khanga to burn it that can take hits from it. Same with Luke and Mawile except said users of said fire and ground also cant take hits from them.
Smogon just doesn't' want to change, they want their meta played their way on their terms. And unfortunately, what that all is, is being decided by the same people, over and over again.
No, its being decided by good battlers who ladder and as thus get voting reqs. Smogon doesnt just decide things on a whim. Of course the aforementioned pokemon couldnt be broken at all. [/sarcasm]
By the waaaay - totally open to another topic haha. Just continuing this tangent because I have a lot to say on it. I DO - have a nice thing to say about them though - the names they give their suspect test threads for things, are kinda awesome lol. I cracked up when I saw the Mega Kangaskan suspect test thread name...