Guess its all about the policy that is why it has not added any new cryptocurrencies into their mix. But, probably it's also because they are just keeping things simple as their company policy. Adding new crypto into their exchange will mean it will be just like every other cryptocurrency exchange which includes all cryptocurrencies into their trading platform which probably is not something they want to do.
No, exchanges want to make the most profit as much as they can on any high volume cryptocoin. But what is stopping Coinbase are the regulators as mentioned in the article.
That means don't expect your favorite ICO token to be listed over there hehehe. But expect proof of work coins like Monero, Ethereum Classic or company backed tokens like Ripple or Stellar.