I don't think that Ben is that passionate a guy. After watching him this season and on Ashley's season, he just doesn't seem like the passionate make out on camera guy. I never saw him give Ashley open mouth kisses and he proposed to her. Compare Ashley's kisses with JP and Ben. He had a few moments here and there, but on the whole it just doesn't seem to be his thing.
I also think that there is a difference between being one of the 25 contestants and being the lead. There is a lot more pressure on you to carry the show. I don't think that Ben had the personality to be the lead and the producers should have realized that before they ever offered it to him.