Again, this has also something to do with size, no matter how often you claim it doesn't: With a small market cap, there is nothing at stake, and even if the community somehow managed to reach some degree of decentralization, miners still don't have much to lose if they adopt some random change. Since there is much at stake for Bitcoin, people do actually care about avoiding a blockchain split. This leads to being conservative about changes that are adopted.

You have to observe these systems as a whole in order to understand them. There is no such thing as an Altcoin that is just decentralized, with no other characteristics being relevant.