Because tumblers/mixerss usually operate under TOR and are by default rogue agents, while exchanges are legal entities with AML and KYL practices, and if not, I wouldn't touch them with a 2000km titanium pole.
Thanks. But Bittrex and Polo don`t give a fuck of who you actually are, right? You can open as many accounts as you want. But maybe you wouldn`t touch them with that pole

Given the fact that the Poloniex front-end is a bogus trading platform due to it's spectacular lag and "ooops, sorry, servers crashed", constant non-triggering orders at critical times, basically feeding the API bots... No, I would not touch Poloniex with any sort of pole, ever more.
They don't "give a carp" as long as you dont trade FIAT, but there is a 2k equity withdrawal limit, so take that into account.
If you are doing funny things, your IP is massively easier to attain and crap loads easier to hunt you down, if they so wish.
But, as mentioned, yup, they can indeed work great as low-level mixers.