a) It's maintained by CitusData which was acquired by Microsoft and so is now both unsupported and based on history at risk of being unmaintained.

b) Is not part of the core PostgreSQL codebase and so you are forever having to worry about compatibility.

c) PostgreSQL still lacks multi-master HA which is needed when you want to HA but continue to scale out.

FWIW, pg_auto_failover was released post-acquisition and is most certainly not unsupported; it's actively developed and new feature releases have been solid. It would be nice for this functionality to be built into Postgres, though.