Coinbase is among biggest and most valuable crypto exchange markets in the world similar to that of Binance or Gemini. Having a market cap of nearly 2 billion dollars Coinbase has long enjoyed the reputation of being one of the largest exchanges that deals with cryptocurrencies. According to the Coinbase team, Ripple will not be added onto their exchange anytime soon. Furthemore, the team leading Gemini was also of the same opinion and sentiment.
For these exchanges, the most important factor for listing a coin in their exchange is to make sure that the coin is maximally safe and secure. It is also a requirement for the coin to be decentralized.
The latter part is what the XRP lacks given that is is not decentralized and hence prevents Ripple from getting listed on these main exchanges. Ripple had been criticized by both crypto enthusiasts and investors for its lack of decentralization. In other words, XRP in a way, got a third party handling operations which is basically everything decentralization is against.
Since Coinbase wants to become the most trusted exchange, getting Ripple listed despite XRP not matching Coinbase’ listed criteria would hinder this objective.