I have been thinking the same and the only other thing that comes to my mind is that STEEM uses DPOS vs POW or POS which many others use. This may have them somewhat hesitant as it may cause liquidity issues. In their Digital Asset Framework (https://pro.coinbase.com/static/digital-asset-framework-2017-11.pdf) they state the factors they will review. I believe I have read here on Steemit that only 18-20% of available STEEM is on exchanges so an addition like Coinbase could strain that and create great imbalances. Just a thought...