Its quite simple really.
There are tons of action games that come out every year, tons of high budget titles to choose from.
There are tons and tons of high budget shooter games that come out every year
There are 2 high budget Stealth game franchises, SC and MGS, they do not come out every year, and outside of these two titles, there is no other significant titles. (maybe you can count Assassins Creed)
Action titles, shooters, racing games etc are mainstream genres. There are tons of varius games that all sell a helluva lot of copies within each genre.
Stealth games? Not so much, outside of SC and MGS there are no significant sales (i dont even know if there are any other titles). Overall, the stealth genre sells very very little software compared to the popular main stream genres.
Stealth genre is niche market, and thus MGS is a niche title.
The genre is definately very small, shooter games probably sell 10x more titles per year than stealth games, if not 20 times more.
The reason why say Halo 3 is not a niche title is because its a game in a popular genre that caters to much more people than stealth games does. How well MGS or Halo sells really doesn't change that.