That is because this is a node policy change, not consensus or protocol. Consensus changes and protocol like increasing the block size limit require a fork, hard fork in the case of block size and soft fork in others like OP_CLTV. Forks require consensus and agreement since those can fork the blockchain. Node policy changes don't since it is specific to individual nodes, not the entire network nor the blockchain. Since this is just a node policy change, nothing is being affected on the network, just whether nodes want to accept or reject RBF transactions.

If you think that this change was too hasty, I'm here to tell you it is not. The subject of RBF has been discussed for years already. Peter Todd has had the code implemented in a fork for a while and this code has been tested and tested and discussed for ages. In fact, the idea of replacing transactions is not original. It was originally implemented by Satoshi but removed due to DoS concerns, which have been fixed by requiring that the fee be raised. Hence Replace-By-Fee.