I did a quick comparison of the power conduit paths from the main reactor to the nacelles to see how 3 of Crazie Eddie's hypothesis holds up. None hold up under examination but the 3rd does suggest that sufficient cargo space - the one common thing among the ship classes that survive into TNG's time is one of the likely factors for their longevity, IMHO.
1. "Prohibitively far" from each other
a. Constitution: 396' reactor to single nacelle
b. Reliant: 326' reactor to single nacelle
c. Excelsior: 616' reactor to single nacelle
The Excelsior is the worst in distance. The Reliant the best.
2. "Occupational hazard for the crew" as in more length = more danger to radiation
*The Excelsior and Constitution limit power conduit exposure to Engineering hull and neck. Reliant runs into the single hull. Constellation also is connected to the single hull.
a. Constitution: 186' reactor to warp pylons. 262' total if include to impulse deck
b. Reliant: 562' reactor to warp pylons.
c. Excelsior: 366' reactor to warp pylon hump. 470' total in include to impulse deck
The Constitution is the best as the warp pylons compose of most of the conduits. Reliant is the worst because of the short pylons and most of the conduit running internally to the main hull.
3. Upgrade friendliness (or future increases in size of hardware)
a. Constitution: Increased reactor facilities could reclaim cargo bay area. (This will lead to reduction in cargo space.)
b. Reliant: Increased reactor facilities could expand into cargo/shuttle bays. Note: warp pylons are almost half as thin as Constitution indicating a possible max limit increase in power transfer to warp nacelles.
c. Excelsior: Plenty of room to expand.
Constitution is the worst as it loses the cargo space of a limited volume engineering hull. Reliant is the most flexible due to her boxy nature. Excelsior has significantly more volume.
Note: I haven't had time to take a look at the Constellations.