I always figured the Constellation-class were easier and cheaper to build than the TMP era Enterprise-class cruisers while using many of the same parts. The Constellations look to be of simpler construction and less elaborate design--hell, it's really just a beefed up saucer section with the stuff normally in the support hull of a Connie shoehorned into the Constellation's main section. The four nacelles are not exact replicas of a Connie's, but serve to provide similar performance in a quad arrangement.
The Enterprise and Constitution classes were the top-of-the-line while the Constellations could have been more workhorses to perform many of the same functions while more of them could be built for less investment per unit.
The variants of the Constellations we saw could have been science survey specific variants bulging with extra sensor systems. Meanwhile there could well have been variants that were cleaner and more streamlined looking akin to the familiar Constitution-class.
The Enterprise and Constitution classes were the top-of-the-line while the Constellations could have been more workhorses to perform many of the same functions while more of them could be built for less investment per unit.
The variants of the Constellations we saw could have been science survey specific variants bulging with extra sensor systems. Meanwhile there could well have been variants that were cleaner and more streamlined looking akin to the familiar Constitution-class.