It's not centralized which means that Bitcoin transaction and network is not dependent on any centeral authority and is operated by all the nodes and miners across the network and even some of them leave it btc would still be functional.It cannot be shut down from any server and you can send instant payments to anyone globally without any third party intervention and that's what is called decentralised currency.

Talking about the prices they are determined by the market forces of demand and supply and when whales dump thousands of coins they are holding it can cause temporary dump in market but they cannot control the network or forge the transactions with any super computer also so that's the main point.The other altcoins which are centralised are controlled by any company can make changes and limit the supply and make rules for you and moreover thier most of the supply is concentrated to small wallets and they can make permanent dumps in market like Dogecoin but it's not the case with Bitcoin.So that make bitcoin different from others which is why it's most popular currency.