I’d like to give a more pragmatic answer to the question.
When answering questions such as “why is there no good decentralized alternative Twitter or Reddit”, or “why is there no good decentralized alternative to Discourse”, and in general “why is there no <insert thing that I would like to use here>?” the answer almost always boils down to: because we haven’t found a good solution to the data storage/availability problem.
In other words, we (as in the blockchain ecosystem in general) haven’t found a good way to store data “somewhere” and being sure to be able to retrieve it later. IPFS for example doesn’t guarantee that you’ll retrieve it later, and Filecoin last time I’ve checked doesn’t really work and has oriented itself towards cold storage anyway.
That’s a very pragmatic problem to solve, and seems to me to be the blocker for most if not all actual real world blockchain-based software.