Because no developer is going to ignore the current 3ds install base and the extra buttons will remain optional for most games like how the CCP is optional for the very few games tha even support it. Developers aren't going to look at the increased cpu power and think "well we could make X times more money making a normal 3ds game, but I can't ignore that extra cpu power so let's forget about being successful!"
Well, it depends. If the current power issues are really a bottleneck in game development, if the changes allowed easier adaptation of current mobile games (although I don't think a simple RAM/VRAM boost could really make that difference,) then sure, I could see them supporting it and ignoring the larger market in favor of something they can produce affordably without stretching the budget trying to cut it down or fit it in. It was eons ago, but the GBC (and a very different scale of upgrade, even though the chipset wasn't at all a generational leap) did have that breakthrough tipping point where it very quickly became the dominant system (look at the library, it's shocking how many Game Boy Color exclusive titles shipped in that system's short run.)
The sad thing with these billions of portables is that the new systems are selling the new games, they never beg to be fed like a console, they often just get their first Mario and Pokemon and a few other things before it goes on a shelf or in their backpack. If these people do manage to get tired of their old games, usually what they reach for is the sequel or the familiar game to what they already own. The "install base" doesn't mean as it's all an active base of customers. (The trick with F4P mobile games is that even if that habit sets in, there's always turnover in new devices and so new chances to get a phenomenon going, plus no barrier to entry and now push messaging and other marketing techniques allow them to reach people who might take a rare chance that they wouldn't for even the small $30-40 that a 3DS game costs.) Nintendo does do a good job of getting gamers to come back for its products, and there are some portable perennials that have a fanbase, but in general, it is a surprisingly difficult market to make headway in.
And the thing about splitting user bases is that when it happens with a portable, when/if it happens, it happens quickly because of that. When Nintendo announced the DS, it was the "third pillar" to the GBA, but by that Chrismas even and certainly early that next year the GBA software had dried up aside from some still-in-development Pokemon games and the odd stray Kirby game (and no, I am nor forgetting DrillDozer.) DS was the most popular gaming system of all time, but again, even the shovelware was dead when 3DS hit and Nintendo did the usual late-term Pokemon/Kirby releases (even if it was hitting a natural slump, the last year of software support was disproportionate to its size.) As I mentioned before, GBC took over (you'll see a hell of a lot "Only on Game Boy Color" banners than you will stuff like Zelda: Links Awakening DX with enhancements when running GBC). These systems, however, were all cheaper than the 3DS today, and there were fewer choices stopping gamers from upgrading their Nintendo handhelds, so the odds are much tougher this time.
The main thing is, there are a lot of games that are NOT being made for Nintendo 3DS right now, despite the system's success. So will this New 3DS system create a market where those games are more attractive and feasible to be made? Probably not (like I said before, the original 3DS is hardly being pushed it its limits so why do developers need more horsepower?) but if there's a tipping point that this hardware leads to a ramp in, it could happen. It'd certainly make Nintendo very happy if this new system gave a jump to software development on its platform, so if it takes off, they'd be fine with leaving mostly behind the old install base to nurture a new one, they've done it before; but then again, it didn't happen with DSi and they didn't push it (aside from on the DSiWare shop) when that didn't happen.
