There is a lot of time and financial investment in having a specific crypto library FIPS certified on a specific platform and ensuring everything works together. For our platform that investment happened to be in OpenSSL. Additionally, on a technical level, the approach taken by the existing dev.boringcrypto implementation is restricted to only linux/amd64, whereas we need more architecture support and flexibility.
Seeing as dev.boringcrypto is essentially an upstream fork, there is more leeway in terms on fitting these changes in and hopefully in the future re imagining the mechanism by which alternate crypto libraries may be used.
Finally, and not to be rude or snarky in any way, but there are Go team members who already work on and maintain this dev.boringcrypto fork. Asking them to review more code for that branch isn't out of the question, and contributing code back to an upstream project is typically seen as being a good citizen of that project. Others have expressed interest in using Go with OpenSSL or another crypto library as well. I think that unless you pay the salary of Go team members you're not in a position to dictate what they should or should not be working on.