First, the militant group’s revenue has been harshly cut due to the dropping price of oil, coalition strikes, and the group’s inability to capture new oil-rich territory. Second, given its falling revenue, the barbaric group has allegedly resorted to selling the organs of its dead fighters - as well as hostages - as a source of revenue. Given the fact that IS captured Mosul in June of 2014, it is certainly feasible that the group, desperately in need of money, had sufficient time to sell the real artifacts on the black market, replace them (or at least some of them) with duplicates, and then destroy the replicas in front of cameras.
I may be wrong but I have hope. I have hope not only because I am an eternal optimist but also because I believe these barbaric individuals’ greed may have overpowered their savagery. To satiate their need for power and money, they may have resorted to practicality in selling the artifacts on the black market as opposed to their typical modus operandi of indifferent and brutal destruction.