2 reasons they are not as popular as they could be.
1) Logistics with the seats. Your rotations have to work perfectly, or you may end up with the seats in one location, and the plane in another one.
2) The seats you have in those planes are fixed on pallets. That means they cannot accept an IFE system.(if you start to have cables and connection, you cannot change the configuration in 20 minutes) And those seats are not up to the standards of some airlines.
ONe good example in France: Europe Airpost (formerly l'Aeropostale) has some 737 QC
for the night .
In the psat, they were used during the day by AIr Inter for French domestic flts. After the Air France bought Air Inter, they used them for a few more years, and decided that the seats were no longer up to their standards. (and they stopped using them)
The good news is that Aeris, who got slots at Orly from the defunct AirLib, has decided to use them to start their low cost operation