Because a previous commenter wrongly said, "the Spanish made a point of seeking out all the Maya books". It wasn't "The Spanish" it were some individual actors clearly acting against "The Spanish" crown wishes.
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.
