Because supercaps aren't new and don't have the energy density of conventional lithium batteries, so they're completely impractical for use in something like a cell phone, unless you fancy carrying around a power pack in a backpack.

The fact that the device has only been tested on an LED (which will draw ~40mW) and is being heralded as a cure for slow-charging chemical battery packs which will draw something like 30x that and have to store something like 5.2 watt-hours of juice should be a pretty solid giveaway.