to go against what morgan has said, i think it is more than just advertising.
Dreams and riu have facilites set up to do weddings 9gazebos, seperate catering dept.). like they have gazebos, a dedicated wedding department. They are also all-inclusive and that is what alot of DW brides and grooms are looking for. Also, along with resorts like palace resorts they offer "free" weddings and will handle arranging the legal or symbolic ceremony.
also, villa del arco is mostly eropean plan. they do have an all-inclusive plan but it is more than what you would pay at dreams.
I think another thing to keep in mind is that people that have their wedding at dreams or riu tend to have a great review upon their return and write about it. a good review says alot more than any paid advertisement.