No one can tell you why your animal died the ONLY way to tell would be to have a necropsy done if you kept her body refrigerated.
Sometimes they just died without any apparent reasons. It's sad but it happens.
Trying to guess here will not get you any closer ti an answer.
Not sure if in the future you are planning on acquiring another BP but if you do try to buy a animal from a reputable breeder, rescuing is a great gesture but if you have limited experience it often becomes a difficult situation.
Addionally make sure you do some research on husbandry which means no hot rock and close monitoring of your temperatures at all time among other things.