One problem is that decentralization sometimes doesn't scale.
It's cool to have completely decentralized email, but once many people use it, there will be lots of spam, and it's hard dealing with that in a decentralized way, I guess.
Regarding decentralized email: there are technical solutions to spam like Hashcash [0]. It has drawbacks, like a bit of energy consumption. On the other side it keeps big corps reasonable (or gives them an upper limit to how bad they can behave), as if they start to behave too bad, such solutions would likely see increasing adoption.