The biggest problem with adding BCH is that it would be extremely confusing to new investors. The majority of Coinbase's users don't have crazy knowledge about the Crypto field - they just want a convenient way to throw money in. Giving someone like that the choice between Bitcoin and Bitcoin Cash would be a huge headache.
That being said, I could totally see BCH coming to GDAX, since anyone day-trading Crypto should be much more knowledgeable about the difference.
If it happens imagine all the people who will transfer BTC to their BCH account and vice versa. What a headache for Coinbase. I guess they can name it BCash to make it more distinguishable.