"Niche" is not a term exclusive to the fragrance market. In fact, I hate that term. A "Niche" is focused towards a small segment of the market. Apple computers are a niche in the PC world b/c only a small percentage of the population uses a Mac. Most people use Dell's, HPs, Gateways, etc...Hybrid cars such as the Toyota Prius and Honda Insight make up a very small niche segment of the automobile market. You see, the brand does not equate to niche. Lots of people own Toyotas, but only a select few people own a Prius. The same applies to fragrances.
There is no such thing as a "Niche" brand, but the market for certain fragrances is much smaller. For example, you can ask a random group of men if they've heard of Cool Water or Acqua Di Gio. Chances are they'll say yes. Now ask that same group about Serge Lutens Chergui or Amouage Jubilation XXV and see who's heard of them. There are companies who make a mainstream scent but also make something less known, such as Tom Ford. He has TF For Men, Grey Vetiver, Black Orchid, etc...which all have ads and are marketed in the mainstream. But when's the last time you flipped through the pages of GQ magazine and saw an ad for Tuscan Leather? Never. Same with Chanel, Dior, and Guerlain.
One misconception is that "niche" things are made with higher quality. That's certainly not always true. As a whole, the term niche has been pretty much ruined by people in the fragrance community. I hate seeing newbs pop up saying "Hey, what's a good niche scent for summer?" Seriously, does it really matter how exclusive that scent is? Forget Creed, Bond No 9, Serge Lutens, and Amouage...you know who's really niche? The 24 the Fragrance lineup. You can't even find them in stores and nobody outside the FragComm has heard of it. Seriously, who goes out looking for a fragrance made specifically for Jack Bauer's tv show? That being said, they're fantastic!