I truly have no idea, but there is probably some simple and logical reason behind this. But check out Steemvoter initiative to get STEEM listed on Quoinex and other exchanges, maybe they will eventually conquer Binance too.
https://steemit.com/steemvoter/@steemvoter/steemvoter-guild-quoinex-promotion-for-strategic-alliance
Thanks for the information. I think STEEM will get more attention after SMT release and price rise. Its volume is increasing. I don't know but there could be some hurdles to setup exchange node for STEEM which may need dev support (i.e. regular outage at HitBTC and Poloniex). However, after HF and SMT, exchanges should be a burning issue for the community & STINC.
I actually need to do much catching up on Steem, thanks for reminding me.