Coinbase doesn't support most other coins either. There are exchanges that are enthusiastic about supporting all of the altcoins out there, but Coinbase isn't one.
That market cap is illusionary because there's no way to deposit BCH into exchanges right now. ViaBTC apparently requires 20 confirmations which would take about a week at the current hash rate, the other exchanges like Kraken and Bittrex aren't accepting deposits at all and don't plan to for a few days yet. So essentially all of the BCH making up that market cap cannot in fact be sold at all right now; the only BCH can be is from Bitcoins that was in exchanges that support BCH at the time of the split.
