I’m not entirely sure I not familiar with it but some explanation is that the external surface of the prion is hydrophobic (rejects mixing polar solvents like water/acids and bases) and as such it is not broken down under the acidic conditions of the stomach. (I could be wrong though as I’ve not read up on it).
Prions are actually very simple compared to viruses and other diseases – they consist of a protein that hasn’t folded up into the correct shape. It’s actually this ‘wrong’ shape that gives them their protection – they’ve folded up so tightly that the strong acid in your stomach can’t find any chemical groups on them to attack. You need to heat them up to over 100 degrees C in strong acid or alkali for them to start breaking down.