I think that it's because it's a matter of personal policy rather than consensus. Each node has a right to set their own policy, because there is no in unified memepool, each node has it's own memepool. Because bitcoin is a monetary network if policies are set liberally then more content would be written to the Blockchain, and if nodes refuse to relay such content then they need to pay more to miners to get into a block, creating a natural policing mechanism.

I see the policy layer as equivalent to the classical legal concepts of malum prohibitum (cheating on your taxes), and the validity layer as equivalent to malum in se (murder).

Its up to nodes to regulate, to keep things hygienic, and miners to validate.

Or it could be that Satoshi didn't think of everything and get everything he wanted into consensus. There is abundant evidence that he intended for bitcoin to be a monetary network and that regulating against too much content getting added to the Blockchain was not considered censorship by him or early Bitcoin Core contributors.

This is one of the best explainers of what OP_RETURN is and how it came about. It was always limited by filters and the filters are effective even though they are sometimes bypassed. https://blog.bitmex.com/dapps-or-only-bitcoin-transactions-the-2014-debate/