If you want (what I think) is the real reason: TPTB probably felt that since the Constitution Class Enterprise was a "hero" ship, there can't be another ship on screen that looked like it, since it could cause some confusion among the audience.
But that's just my two cents worth.
I think that can be partly traced back to Gene Roddenberry, who vetoed the notion of an entirely new class representing the Enterprise in TMP. He seemed to think this would cause the audience to "forget" about the TOS incarnation.
To further confuse my earlier point, in TOS, whenever they showed another starship, it was almost always another Connie.