From Howling Tower: Complexity and Option Fatigue « Kobold Quarterly, by Steve Winter (ex-TSR, ex-WotC):
In the late 1980s, TSR studied the sales appeal of AD&D compared to “Basic” D&D (Basic/Expert, B/X, or BECMI, as it’s now known). The common wisdom was that Basic D&D was the best choice for beginning players because it was a simpler game—it had fewer rules. New and inexperienced players, however, actually saw that paucity of rules as a drawback. To them, Basic D&D was more complex than Advanced D&D, not less, because the DM and players were faced with more situations that had no clear solution. In contrast, AD&D told you exactly (or approximately) what to do in an enormous range of situations. The answers might be hard to find, but they were in those books somewhere. Extensive rules that covered more situations translated to the DM spending more time flipping pages in a search for answers but less time sifting through options. The end result was a perception that while AD&D had more rules than Basic D&D, all those rules made it easier to play. Inexperienced players liked the confidence that came from AD&D providing all the answers, so they gravitated toward AD&D. Experienced players liked having open-ended options and were the main audience for Basic D&D.
That result surprised a lot of people inside TSR. It was an eye-opener, and it affected design and marketing philosophy for years.
See also the comments thread.