I'm guessing that his 1st person description of the human sacrifices carried out by the Mayan and establishing a connection between those and the need to erase the culture that enabled them and that he - wrongly or not, we can't know anymore - saw as enabled by those books had some weight there...

The Spanish crown didn't have in mind to destroy other people books, but then again, they also didn't have in mind that they casually, recurrently and nonchalantly offered human sacrifices to their "gods".

Probably the order of priorities for the Spanish crown was books < human sacrifices.

Strange times, those, eh?