I don't think that the "site" has anything to do with doing well traffic wise. A site is nothing more than an address on the "Grid" called the internet. Traffic and "doing well" are related to the value of the information given and the methods used to market that information.
Niches are small areas of information which focus on limited information. It can be beneficial to market a niche, but that is not necessarily related to a "site".
You could market your "Niche" on Yahoo, Google, Hubpages, Helium and a variety of other sites and see similar or very differing results.