Retractors are not DIR because if you use them you will immediately die a painful screaming death.
Seriously, a retractor is not needed, and DIR divers don't bring along what they don't need. A retractor solves a non-existant problem. Unclipping and reclipping the bolt snap takes little effort with practice.
Also, if something were to snag on the retractor, it may not be immediately apparent to the diver as the retractor may spool out line when snagged. (If the retractor is equipped with a "locking mechanism" and doesn't immediately spool out line when locked, the lock may jam at an inopportune moment, thus adding another potential failure point.) With an SPG attached using a bolt snap, the snag would be immediately apparent. This is pure speculation on my part as I have never used a retractor for my SPG. But I have snagged my bolt-snapped SPG when I erroneously configured my left d-ring too low on my harness.
Mike