Why some cryptos, such as Ripple are called centralized ?
In some sense, the Ripple protocol is decentralized. That is, if Ripple (the company) disappeared today and took their validators offline, the network would still persist. However, as it stands, Ripple (the company) is essentially a central authority with significant control over the network.
I would consider it highly centralized. It depends on a trusted network of validators.
As one commenter here points out:
If it’s the same group of untrustworthy parties doing the validators, then Ripple becomes more centralized. Likewise, if those validators are separate, trustworthy parties, then it becomes less centralized.
Also, XRP is not mined. It was centrally issued by Ripple (the company) and the supply is highly concentrated under their control. I think they control more than half the supply. That's a big part of why people call it centralized.
If it's using the blockchain technology, as from I understand, it should all be decentralized, so no one is in charge, that's why i'ts safe. The community and the system works by itself.
In some protocols (like Ripple or masternode coins), not all nodes are created equally. If Ripple (the company) wanted to censor a bunch of transactions or fork the protocol to new rules, I'm sure they could.