if Bitcoin were to become centralized, it could raise concerns about the control, and the investors might loss confidence in it's ability to remain a reliable store of value and a medium of exchange. centralization could also make it suspicious to government regulation and restrictions, which could further impact on it's value.
in conclusion, Bitcoin value is derived from its decentralized nature, which is closely with it's ideal, and any move towards centralization could fundamentally alter it's core and diminish it's value proposition.
Bitcoin has centralisation too: new protocols are created by a small group of people. It doesn't affect data storage, since blockchain is present on every mining tool, but it affects the development of the overall system, and also it has a impact on the adoption progress. Moreover, after some time BTC can be centralised not only by means of controlling update centers: the public acceptance of BTC can create menaceful and greedy money-controlling cryptotycoons out of several hodlers, just like we see it with dollar now.