Here is another point of view that makes sense to me and is quite likely:
The assault on antiquities enacted in the recently released video footage is part of ISIS’s objective to destroy anything—from humans of different beliefs to the cultural legacies of ancient civilizations—deemed idolatrous and outside of the extremist group’s narrow interpretation of Sunni Islam. Given that looted antiquities are one of ISIS’s sources of funding, it’s possible that in this new video, the militants were just destroying whatever was too big to smuggle out, as journalist Sam Hardy suggested on his cultural heritage blog Conflict Antiquities.
However, Brian Daniels, director of research and programs at the Penn Cultural Heritage Center, cautions against underestimating ISIS’s ideological aims.
“We should not confuse archaeological site looting with ISIS’s commitment to destroy artifacts in such a way that it can realize its millennial beliefs and score a propaganda victory,” said Daniels in an email to Bible History Daily. “We have seen that ISIS is willing to destroy portable objects, like valuable antique books, for example, that could otherwise be sold to collectors. Many commentators have observed that ISIS is media-savvy. I think what we are seeing is that ISIS believes that the destruction at Mosul is more valuable to their core interests than money.”
The rest of the article is a good read.