I don't know this to be fact, but I suspect Coinbase is in the business of delivering well established coins for fiat; they don't appear to have any interest in being in the speculative business.  Their platform isn't suited for fast and frequent trades so they appear to build around coins that have huge volume and market cap.  Lisk looks like an awesome project and may one day be in Coinbase but it'll be when it's fully established and has relatively low volatility (for crypto at least.)  For context, the three coins they support are:

• Bitcoin: 2009
• Ethereum: 2015
• Litecoin: 2011