Firstly, people are becoming richer, they have more money to spend.
Secondly, products nowaday are more diversified not only in appereance but also in purpose. So people now have more choices. You can see that cellphone company continually introduce their new design with new services. Or tendency of fashion changes according to time.
And people don't want to be left behind. They get bored with the old ones and keep their eyes on the newer. Especially for the rich, who have so much money to spend, they always like unique things. To some extend, shopping is a way to show how generous they are.