I've written my response three different ways, but it keeps coming down to you have assumed information that hasn't been shared. I didn't advocate two transfer fees, and I don't know where the second one came from without making assumptions about fiat gateways, in/out transactions, etc.
Blocktrades isn't a fiat gateway, so it would be cheaper to trade across two currencies on poloniex than across one on Blocktrades, unless Poloniex's STEEM withdrawal fee is abnormally high (on binance it's 0.01 STEEM). If their withdrawal fee is high, maybe that's why people weren't trading the pair?