The reason is simple as AMD and Intel have x86 and Nvidia doesn't. Nvidia has Tegra 2, and in late 2011 Tegra 3, so it hopes to capture a piece of market in tablet and dual core phone markets.
Nvidia sees superphones as computers first and phones second, and of course there is Flash support. Nvidia also hopes that games will look better on Tegra as well. After all, Nvidia has The Way It's Meant To Be Played and we all know that scheme proved quite succesful in the past.
Oh yes, Tegra 2 can accelerate flash in hardware. Five years after the Tegra adventure started, you can see some actual design wins that could make some money and fill the chipset void.
