Cost is the thing. Too high a market value of the plane before conversion will limit their prospects to passenger carriers. The basis for bringing an aircraft onto the cargo fleet is that the total cost (combined purchase, conversion and any necessary maintenance) is low enough to be able to offer a low enough lease rate so that the cargo operator can make money.
Unsuitable aircraft for cargo include earliest variants (where the weights are lowest because the designer is at their most conservative for the earliest planes into service), those with lower maximum take-off weight (MTOW), and with payload-range restrictions. The oldest aircraft are less desirable because they have a limited operating life. However MTOWs can often be increased during freighter conversion by incorporating service bulletins that might tip the balance for a particular aircraft type towards conversion. A higher MTOW improves the operating economics of any freighter.
And a significant catalyst for any successful conversion programme (pax to cargo) is the availability of meaningful blocks of standard aircraft, and not just low values. There are usually great (and low value) aircraft types available, but they are in units of ones and twos and not worth the cost of the convertor setting up for the conversion.