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.
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?