^ Inspired by a reference from Mr. Scott's Guide to the Enterprise, implying that a similar thing happened to the Constitutions in the primeline. According to that book, the reason we see so few Constitutions in the refit configuration is because by the 2270s only two of them were LEFT, and all the others were destroyed or lost on exploration missions.
A bit of background: most of these losses are fates occurred or almost occurred to the Enterprise herself:
Sutherland ran into Flint The Immortal, who then shrunk their ship and banished them as punishment for their science officer hitting on his latest android.
Hathaway was shut down and crashed when the crew went spore crazy
Exeter was destroyed by biological contaminants from the planet Omega
Bonaventure got stranded in Elysium
Intrepid was eaten by a baby space amoeba
Defiant interfaced into the Mirror Universe
Constellation made a Kamikaze run to take out the Planet Killer
Lexington, Enterprise, Yamato and Potemkin were zapped by the Organians
Excalibur is on a pilgrimage to find the Borg
Space exploration is dangerous business.