Because in the case of Christianity as they've interpreted it, it completely washes away their past choices and mistakes.

It also permits them to present an olive branch to those most likely to exile them from mainstream society. Combine the conversion with a diatribe against the evil of porn, and you've got a poster-boy/girl reformed sinner.

So, now they can try to get some of the attention they craved when they were "adult stars" from a different type of audience.