Sometimes these mysteries reveal distortions. In the UK you can lease new executive German cars more cheaply than you can run a second hand one, but that's because the manufacturers will sell cheaply to fleets but not to you. They want to keep residuals up to use the retail market to subsidise the fleet market. It's not simple economics; it's a deliberate distortion and if you understand it, you can appreciate if and how it might unravel to your advantage, or disadvantage. Then there may be tax reasons.

Current series aircraft always look to me about the same as old ones. I can't believe passengers can tell the difference if I can't. The insides are refurbished on a regular basis - it's always going to be cheaper to do a refurb than buy a new hull.

Since it would appear utilisation is not the key, I can only think it's to do with financing. Maybe it's the same as cars. Big airlines can afford to run new stuff because they get deals that are deliberately denied to the outsiders to keep up the desired pattern of resale values. Incidentally in the car world it's not to do with straight volume discounting, otherwise people would pass on the savings to some extent. It's more like a cartel.

However MTOWs can often be increased during freighter conversion

and, for amplification, if the intended scenario involves shorter range, on a medium to longer range hull ... where MZFW becomes limiting .. rescheduling the fuel usage can result in an STC to increase the MZFW to the point where MLW becomes the limitation more often than not .. which can help the payload materially over a shorter sector operation ... eg Ansett did this with the Electra passenger to freight conversions many years ago as a lot of the operation was shortish range (Australian east coast) ... generally TOWs were comparatively low (and the bird went like a cut cat - chaps ought not to be allowed to have so much fun) .. but then we would head off to NZ or similar at max gross (around 52T if I recall correctly) and the old girl tended to be a bit of a dog.

old aircraft in passenger service
 

twistedenginestarter,

at least one passenger carrier – Allegiant in the USA – uses rather old aircraft (MD-80) and a low utilization strategy. And Allegiant is profitable although with a different business model compared to other LCC.

SEC Info - Allegiant Travel CO - 8-K - For 11/16/07 - EX-99.1
(browse down and you find more info why Allegiant has chosen the MD-80 family)

Having been tied up with DC9 family freight operations in the past, the numbers just stack up for that sort of payload region ...