Well, let's look at it this way:

There are several types of amino acids that join together into long chains to form proteins. These amino acids are either hydrophilic or hydrophobic. This hydrophilicity is what gives proteins their shape. Because the body is mostly made of water, the chains will arrange themselves in such a way that the hydrophilic aminos will be as close to the water molecules as possible while the hydrophobic aminos will do the opposite. Additionally, during tertiary folding, separate chains of proteins come together via hydrogen bonds (ref. high school chemistry).

For prions to be able to misfold other proteins, it would need to overcome these chemical properties to replace one or several of these amino acids to cause a change in how the other protein folds.