Coinbase custody is digital assets custody for institutional investors.. Worth mentioning just for storage NOT trading. But that does not mean these coins are not considered for trading in the future. The fact that they are considered for storage also speaks for their possible future trading..
Looks to me like they have added the coins in the order they will add them..Lot's of quality coins all the way down to row 3..where it stars to get weird.
Anyhow, great news for Steem!
https://blog.coinbase.com/coinbase-custody-is-exploring-a-range-of-new-assets-e700496844be