If you look closely on the Blog they posted on their Website it seems there are a lot of Terms and Conditions applied to them. For example, they are looking into these Cryptocurrencies but they may or may not add them. Then again if they add them you might not be able to trade them, You may only Buy and sell them and there are more where these came from.
What I could summarise from all that is that Coinbase may be wanting to clear out any FOMO others may create plus they are leaving a room to go back to if they are unable to provide support for any Cryptocurrency in that list. Long story short they are playing it smart and I like that.
