The operator elects to use the aircraft the operator elects to use. Why concern yourself what aircraft that is?
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.