The case to be made is this: Landry Jones does not look like an NFL QB at this point. He'd be useless to hang on to even as a 3rd stringer since it would be disastrous if he actually were to play. Instead, why not have a guy that could bring something to the table if called upon?
Terrelle Pryor has long intrigued the Steelers, he's the same age as Landry, and he's won NFL games- I doubt I need to remind any Steeler fan what he's capable of.