Why do some people buy used cars, and some buy new.
It's based on a finances. I can name plenty of Cargo companies that buy new...Cargolux, Fed-Ex, UPS, Lufthansa, Atlas etc. And I can name plenty of pax carriers who buy junk...AirTran, PanAm, Omni, Spirit, Carnival, Trade Winds etc.
It all comes down to what you can afford. No other reason.
Boieng estimates another 20,000 freighters by the year 2020. Most comming from the used market.