Well we aren't sure that it did have a single origin. But also, anything primitive enough to generate randomly is going to be viable food itself for other already existing and more evolved lifeforms, and the conditions for life to start by itself might just be a smorgasbord of energy for existing life forms that could out compete more primitive forms or alter the environmental conditions enough to prevent more life from forming. It might just not be possible for new life forms to arise in a non-sterile environment.