Scuttlebutt's pubs are only dumb caching pipes, whereas a Mastodon server is actually the source of your content. That's the fundamental difference between the two.

As every Scuttlebutt node is equal to every other node, that makes it a distributed, rather than merely decentralized, protocol.

It's difficult right now, but the clients are very much under development—DHT is being worked on in Manyverse for example.

The important thing is that the protocols are not designed to be reliant on pubs in the long run.