Because it's not truly decentralized, it's hybrid. The good thing is that your wallet is actually yours and you control it but once you send your tokens to the IDEX contract just remember that books are centralized (or else you wouldn't sell/buy hella fast).
You know which exchange/platform would be good for SXDT? Bancor
I think non of exchange will be good other than their own DEX (Code name: MoonDEX) which will be launched soon. This is because there is a risk of getting delisted SXDT from any exchange as it is treated as security token like IDEX did. Spectre is introducing this DEX primarily to alllow traders to trade SXDT and SXUT.