I think, to answer the main topic - Loreal, Maybelline, and Clinique corner the market, in approachability, ability to obtain product, and ease of use. There's no technique really needed with these brands, you don't need a lot of expensive applicators (brushes) and probably a good 50% or more of women love these brands and keep going back for that reason.
Now - on lah_knee's point of MAC trying to reach all sexes, all ages all races - probably a good 90% of MA's I have met (and I have met a LOT working in the industry) all LOOK THE SAME. Some are bigger, there are some men, some are smaller, some are black/white/hispanic/etc., but they all look edgy, artsy, what have you. I think that this is MAC downfall right now, is in the hiring. They need a wider demographic if this is what they want to achieve. Like, seeing a mid 50's woman with a small brush roll, in a black pantsuit and nicely coiffed hair, with some pretty simple makeup on, would be refreshing to me, and probably bring in a lot of clients closer to her age range. For example, how well do the icon collections REALLY do for the younger age set? Probably not as well as, let's say, Barbie for MAC. I'm not saying that ALL MA's look the same, I'm just saying (and I probably speak for a lot of people) that I generally see the same kinda look from the MAC MA's , with little variation here and there. And it shows in the clientele they attract.