Each "exchange" is fundamentally a business and to do well in business you must choose a part of the market where you can focus. Coinbase appears to have chosen to focus on providing exchange from fiat (US Dollar) to the dominant cryptocurrencies. Many users will buy their BTC or ETH there and then transfer to their own wallet (paper wallet, hardware wallet or web based wallet). Or they may transfer their coin to an exchange that offers alt-coins. Major exchanges like Bittrex and Poloniex broker hundreds of different coins.