To answer the OP, I don't know why you dropped out of the target audience, but as someone that has been dropped and regained several times from the D&D target audience, I can tell you why that happened to me. It might answer some of the question:
1. Later sets of Basic D&D lost me because it was too little, too late to compete with new shiny (1st ed.)
2. 1E grabbed me because I was a teen, it was shiny, and you could play an elven rogue or even multiclass as something besides an elven fighter/wizard (aka "basic elf").
3. 1E lost me because once I had the main books, I didn't much need anything else. And besides, 2E was a little too fluffy for my tastes at the time. And then I got seriously interested in non-D&D games.
4. 2E grabbed me because some of the supplements worked pretty well in my Fantasy Hero campaigns, even if I never bought or ran with the 2E rules.
5. Basic grabbed me again, because the RC was just that good. Still, there is only so long I can play with "elf" as a class.
6. 3E grabbed me because as much as I enjoy Hero, I needed a break from the character point accounting, and 3E looked like a better fit for the particular style of game I was running at the time.
7. 3E lost me, because too much of the early supplements didn't do it for me, and AU/AE did. And I began to seriously dread prepping a game when the characters were over about 9th level.
8. 3.5 never had me, because AE did some of the same things, only better (IMHO). Also, too much of the later 3.5 stuff seemed kludged onto 3E--both going too far, but not far enough, at the same time.
9. 4E grabbed me primarily because of the dread of 3E prep (see #7). There are other things I like about it, but that was the thing that got me to try it right away. Well, that and the Amazon sale, when I hadn't bought anything new and shiny in ages. There was some unspent 3.5 money burning a hole in my pocket.
Stuff I didn't like in 3.5, when glued onto 3E, I now like when it is completely integrated into the whole ruleset.
10. All during this time, Dragon and Dungeon were working to lose me. It took Dungeon several changes of editors and the better part of 40 issues to lose me, but they managed it in the end. Pazio had already finished me off, but the DDI was the wrong way for WotC to get me back.
As to when I"ll be lost and regained again into the D&D fold, I don't know. Maybe Hero System 6E will knock my socks off, since I skipped 5E as, "too many nitpicky changes that manage to invalidate my copy of the rules without solving the issues I have with the game." Given the stated preferences of the current Hero owners, I doubt it, but anything is possible. Or maybe I'll have a retro Runequest itch. Or maybe I'll get so enamored with my Burning Wheel campaign, that I forget about everything else.
I apologize for the length, but I think this illustrates nicely those "fuzzy margins" that Charles Ryan was discussing. It would be incredibly difficult to keep only me square in the sights of the target audience, even if WotC tried to do exactly what I want, all the time. The real miracle with any solid design is that it attracts a substantial audience at all. Being more of the "glass half full" type, I'll suggest that 3E/AE keeping me entertained for the better part of 8 years is more impressive than that I got tired of it eventually.
A lot of "lost me" is really "went onto something else when I'm not done with it".
