why not just use a single finger on the touchscreen, or tilt the device? I think the flaw is expecting the phone to behave like an elaborate 8-button hardware controller.
Single finger on touchscreen works - but tilting I think largely doesn't work. Device tilt was the dominant mode of input for earlier iOS games back when the App Store was new - in part because of the Wii's popularity at the time - but didn't stick.
Part of it's because despite Apple's pretty spectacular accelerometer, detecting fine motion is very difficult, and so the amount of "resolution" available to you as a dev is pretty limited. Variation in fine motor control between players also means that you can't necessarily rely on players being able to affect small, precise movements. Motion controls ultimately ended up being low resolution and relied on substantial/large movements of the device.
This is worse on Android, where the quality of the accelerometer varies widely depending on hardware, and getting consistent experiences across a range of devices is nearly impossible.
Add to environmental factors (say, playing the game on a bus, train, or airplane) massively screwing with your accelerometer readings, and the propensity for mobile games to be played in those exact places, and the situation gets worse.
In any case, this leads to fatigue. Turns out holding your phone up and tilting it every which way is a bit tiring.
tl;dr: Because of individual biological variation, hardware limitations, and environmental noise, fine motion control for mobile games didn't work out, at all.
