Well, my guess? Testimonies aren't just tools to try and convert non-believers, they can also serve as affirmations of faith to the believers ("look, God has saved this person and totally changed their life, God is powerful/good/etc and I'm on the right path!") - for either, the more dramatic stories are what people want to pass around. The sudden conversion fits that criteria, so either the stories get embellished to make the conversion more sudden than it was, or these people really did convert quickly (not unheard of - some do convert quickly and never look back - one of my friends became Christian and burned all her old pagan books and whatnot...) and since it makes for a better story, they're the ones more likely to be passed on. Could say the same for the standard sex, drugs, drinking line that comes up so often.