My theory is that although the Constitution refits look nice, Starfleet found them lacking in some way. That’s why Enterprise was relegated to a training ship so soon after receiving a major 5 year long overhaul, why a derelict Enterprise A was available to assigned to disgraced Admiral Kirk on short notice, and why we never see a Constitution in TNG or DS9 despite the Miranda, Excelsior, and Oberth classes still being ubiquitous, with even the occasional Constellation being seen in service.