Is there any reason for an alternative Bitcoin implementation to use Berkeley DB?
Backwards compatibility is a massive priority for Core and it is generally very conservative in its technology choices. I don't think it would consider a new (database) technology until it had been proven over many years on other projects. It is providing a consensus engine and infrastructure and doesn't have the same tolerance as startups or bigger corporations for cycling through different options and experimentation if it is going to endanger backwards compatibility or require many developer hours to revamp shortly afterwards.