IMHO this is not justified but market reality.
I have been working for a company operating turboprops and jets for a long time and have seen lots of turboprop pilots transitioning to jets (both within the company and to companies accepting turboprop pilots for jet positions) without any problems, including Dash 8-300 to 747-400 (CargoLux).
Turboprop pilots should typically have better skills in manual flying and flying without autothrottle (as none of the current turboprops have ATS) which might make them more attractive to companies if the emerging trend towards more weight for basic flying skill should continue.
The requirement for jet hours might be linked to an expectation that jet pilots would typically be familiar with advanced flight guidance/FMS (not true for the older CRJs unless covered by minimum jet tonnage requirements by some operators).