Do you really want to bring up a comparison to Char-Y? Because Char-Y is a far more versatile mon than Crawdaunt. There's the fact that until it Megavolves, your opponent has to guess whether you've got a CharX or CharY. Then there's CharY's immense sweeping abilities and very limited list of counters.
I'm not saying Crawdaunt is a bad mon, it's just that a lot of mons look good in a vacuum, and Craw is just outclassed as a Water-type, as a Wallbreaker, and as a Knock-off user by Keldeo, Azumarill, Rotom-W, Kyu-B, Char-X, Talonflame, Bisharp, and Mandibuzz. Yes, Crawdaunt is the only one that fills all 3 niches at the same time, but I don't think it's worth bringing a mon that's mediocre-to-above-average in all 3 roles in OU when you can bring two mons that excel at those roles, and bring added value.
1. Charizard Y is a wall breaker. It can't sweep with 100 base speed. I totally agree about its versatility though.
2. Crawdaunt has a lot of niches that prevent it from being outclassed as a water type in general. Does Azumarill outclass it? No, they have totally different niches. Crawdaunt hits harder, and has the most powerful knock off in the game, while Azumarill can sweep with +6 priory, and has bulk.
3.
WHAT? Neither Keldeo, nor Rotom-W, nor Kyu-B, nor Char-X, nor Talonflame can even learn knock off. Azumarill can, but Crawdaunt's is twice as powerful, and Azumarill can rarely fit it in its moveset. Your point about Mandibuzz is fair, but Mandy doesn't use the move offensively at all, so I don't truly think it's outclassed. If it were still 20 BP, yes, Mandy would use the move better, but it isn't; it now can be a hyper offensive move, which is how Crawdaunt uses it. Bisharp is your best comparison; both are frail, relatively slow, very powerful, and use Knock Off offensively. The differences are that Crawdaunt has a Knock Off that is around 1.3 times more powerful than Sharps and Crawdaunt reliable priority, while Bisharp has more powerful priority, better speed, an arguably better defensive typing, and Defiant.
4. Teams can often need a pokemon with many good traits. If you want a hard hitting water type, an offensive pokemon with knock off, and a wall breaker that eats chansey for breakfast, you have little reason not to use Crawdaunt. Sure, you could make a team of Mega-Mawile, Azumarill, and Conkeldurr to do that, if that's exactly what you want, but why would you when you a) Have three physical attackers with little synergy, and b) could take up only one slot by using Crawdaunt?
Yes, Crawdaunt has flaws (bulk + speed). But, it has big niches, and isn't outclassed at what it does by anything. If you try to make it do something it shouldn't, like sweep with Dragon Dance, yes, it is pretty outclassed. But when used as a wall breaker with the most threatening Knock Off in the game, plus the equivalent of an 80 BP priority move, plus the hardest hitting physical water move in the game, plus great coverage with superpower, nothing outclasses it, and it should be, in my opinion a low B ranked (a.K.a B- ranked) Pokemon.
EDIT: If what Memoric said is what you meant, sorry :P
But, Keldeo and Rotom-W are special, Talonflame can be a revenge killer or a sweeper, and Charizard-X can be a tank or a sweeper. Crawdaunt is a physical wall breaker, so I don't see how it's outclassed. Kyu-B is another wall breaker, but it's different from Craw. It handles Rotom-W and Venusaur, who obliterate Craw, while Craw makes quick work of Chansey, who walls Kyu-B. To be honest I think they'd work well together. (Also, sorry if this sounded mean. It wasn't meant to be :P)