This is an interesting question. I have two young neices (7 and 5) and they have both gone through the "wanting to be a princess" stage. I dread, though, the day when they want to be Jordan or Victoria Beckham and I shall do all I can to remind them that real princesses do exist and are much better than A List stars!
There have always been many fairytales and stories about princesses (such as Cinderella) and these magical stories have always been told to girls and been extremely popular with them. Parents generally want the best for their children and want their children to be the best and so they tell their girl she's a pretty princess, which will have lead to someone realising the marketable value of the "princess" brand, which then turned into a rather tiresome commodity. Don't forget that as soon as one child has a plastic pink tiara to make them look cool, they all want the same - it's just fashionable (like ponies and horses used to be). It is similar to the fact that most items for girls such as clothes, hair-bands and pencil cases can only be found in various (and rather disagreeable) shades of pink. It's cheaper to limit colour choice and make everything one colour.