Because that’s not what TAI is designed for. UTC is for civil time keeping, and that’s what everything except highly-specialised stuff is tied to. The ISO 8601/RFC 3339 serialisation is UTC-based, so you’ll have to go heavily non-standard to use TAI, and you’ll find a complete lack of support in general date time libraries, so 37 second errors are sure to crop up all over the place if others ever touch things.