It's because XRP is not even a cryptocurrency. It's just a digital token offered by bankers. It doesn't even belong on the "cryptocurrency market cap" if the name is accurate (although this can be arguable for many other coins, but especially XRP).
Just because it doesn't share all of the characteristics of an "ideal" crypto, doesn't mean it's not in the same family of digital currencies. In fact, what matters is if the predominant public opinion considers it a crypto. Then, it certainly has the potential to steal market share from ETH, BTC, or any other coin.