Popularity has to do in part with how many of the aircraft built have become available from the pax market.
This helps to explain the popularity of the 727 and DC10.
Same has happened in the past few years with the A300F4, and is now starting with the older B757 aircraft.
737-300 still far too popular and economic as a pax aircraft, and likely to still last for long in that market. Not to be found into freight market soon.
Hope this clarifies.