Islamic law forbids the depiction of animals, gods, or people in general. The stated reason for this in the qur’an and ahadith is that “god” apparently finds humans trying to mimic his creation through art or drawing as blasphemous.
Exactly how stringently this is carried out is dependant entirely on the school of islamic jurisprudence one follows. At their start, the Taliban were primarily influenced by the Deobandi Hanafi’s, and were liberal-ish ( for islam). After they took power they started coming under the influence of Hanbali Salafists like al-Qaeda, which is when stuff started getting blown up.
Most of the local residents around the ancient sites were pissed, especially around Bamiyan, but mullah Omar was getting way to much pressure from the hardline radicals.
Still, it’s not a total loss, they did find caves hidden behind the Bamiyan Buddhas, filled with troves of artifacts and writings.
As a further thought, hiding a bunch of sacred stuff in caves covered by giant stone Buddhas that you have to destroy before you can access whats inside is probably the most Buddhist thing I have ever heard of.