Using the terminology most commonly found in the crypto space, email is not decentralized but rather federated. This is because participation in the email ecosystem requires permission from other participants in the email ecosystem. If only 5% of the ecosystem is willing to forward your emails, most of your emails aren't going to make it to their destination.

Conversely, in the blockchain space anyone can participate in the blockchain. If a miner refuses to mine your transactions, that's profit that another miner can pick up. And if worst comes to worst, you can set up your own mining infrastructure and mine that transaction yourself - if the system is working correctly, you never need permission from someone else at all to participate. (there are some assumptions here, and if you choose to reject those assumptions as valid you can reasonably claim that blockchains aren't decentralized either)