People arent buying android phones because android is "open". Maybe techies do but everybody else is buying them because they have better features, bigger screens, more powerful, more customizable, better reception, better plans on more carriers...
They are just better devices. Just like the ipad is just better than most tablets right now.
Most people buy Android buy it because it's available. It's the same reason why two years ago they were buying Blackberrys, or four years ago Motorola. They heard about smartphones that can access the web anywhere and their provider of choice have a phone that does that, and hey it's cheap! (if not free) Some of them may not even want web access, they just want a phone where they don't have to deal with T9.
The majority of the world is not technically inclined. Telling them they can customize more, or something is more powerful means nothing because they aren't going to understand where to begin. Same as telling them it's open source. It's over their heads.