Let's encrypt is not some code or even a company that you can split into different branches. Their existence is one based on trust relations that let's encrypt has with browsers and operative systems. It is in one part similar to both domain names and IP address space, in that the technical aspects of creating alternative roots is almost trivial in comparison to getting the trust that is required for an alternative root to be accepted by the rest of the world.

Let say someone created an Russian Let's Encrypt. It has all the technical aspects as regular LE in that you can request a certificate and get one through an acme challenge. That is all great and all, but no browser will recognize it as valid. No operative system will recognize it as valid. The Russian state might add the new LE as valid for government computers, but the real work would be to get any other participants in the world to do the same. The issue is not a technical one but rather a social one that is built on trust.

When Russia invaded Ukraine there was a major discussion if IANA/ICANN should have disconnected Russia from domain names and IP addresses. That discussion ended on a decision to not do that because the symbolic benefit was deemed minor compared to the harm to the system in large, especially once the war end. If you got two roots, then a domain name or IP address can now suddenly have two locations, and it would be a massive pain to try fix it even if people wanted to fix it. Certificate Authorities do not share this trait since there can be an almost unlimited number of roots and none of them can conflict with each other (assuming no hash collision). If Russia spins up a new CA then people can use that one today if they want to, and they can continue to do so after the war has ended.