Monero probably isn't going to be added to Coinbase anytime soon.
I was listening to a talk from Charlie Lee (who works at Coinbase). He was talking about LTC and the potential of getting it added to Coinbase. Anyway, he mentioned that LTC volume on GDAX (exchange owned by Coinbase) has to be consistently high enough before Coinbase would consider adding LTC.
Monero isn't even listed on GDAX. That will probably have to be the first step.
Why does Coinbase only list coins that have high volume on GDAX? Cause they don't want to rely on / go to a 3rd party exchange to source their coins.
Not sure what you are talking about. Litecoin is on Coinbase as well as GDAX. Maybe it was added to GDAX first, but it doesn't really matter. If you create a Coinbase account, you automatically have a GDAX account with same banking info for funding. Only difference is GDAX operates more like a Poloniex type exchange - you place bids and asks, while Coinbase gives you a simpler interface / flat rate based on total being sold or bought. And Coinbase has higher fees.
So maybe they listed it on GDAX first until they had enough volume to be able to manage the Coinbase flat rate. But, regardless, listing on GDAX would be incredible news and open an easy dollar purchase option for all Coinbase / GDAX clients.
Still, I'm skeptical of it happening anytime soon, given their IRS litigation.