Maybe nobody wants to spend effort maintaining it? I imagine it's simpler to build your own S3 alternative than maintain minio.
Also nobody wants to test AGPL liability in court: https://www.reddit.com/r/minio/comments/1fnuv46/does_interac...
Also think there is an abstraction mismatch with the object stores that store the objects with a 1:1 mapping into the file system. Obvious issues are that you only get good listing performance with '/' as delimiter and things like "keys with length up to 1024 bytes" break.