I find your assertion to be flawed.
The Halo franchise, as a whole or on a game-by-game basis, is not really what I would call a masterpiece. A better word, I think, might have been "flagship", as that is a non-subjective term that doesn't connote the supposed quality that not all parties agree upon.
Furthermore, you assert that "The gameplay mechanics are polished to a mirror sheen, are easy to get your head around, and have a surprising amount of depth." Simply put, this is an opinion. Furthermore, there are several glaring gameplay problems, varying in each game, and as we have no clear indication to what game you refer, I won't take the time to point them out.
Further down the line you insist that the Halo games MUST be great because of it's high purchase numbers. A HUGE logical fallacy, sir. The same logic can be applied to "Hannah Montana MUST be hugely talented, because so many people buy her albums and merchandise" or "Hinduism MUST be the correct religion, because there are far more Hindus than Catholics" (No offense meant to any Hindi Escapists, and good Catholics will take offense no matter what I say).
Now, in my own opinion, HALO was a rather good game, if a bit too long and suffering from repetition. Halo 2 had a ridiculous number of glitches and bugs, but it WORKED and it had plenty of hunters for me to bullfight. Halo 3 was essentially a cartoony graphical update to Halo 2 with some slight physics changes that in many cases improve gameplay, but not all. I haven't played ODST yet but will eventually due to the immense nerd-on I have for Nathan Fillion.
The story is also heavily cliched.
I like Halo