Because the underpowered macbook air has demonstrated that "Good enough" performance can be greatly outweighed by ultra portable, power efficient devices.
Cutting costs, increasing power efficiency, and decreasing physical size are all good ideas in my book.
It's a new world out there. People are finally expecting software to run faster while requiring less. Each version of Android has noticeable speedups on identical hardware. Snow Leopard freed up 6GB of storage when upgrading from Leopard. Inefficient software no longer buoys hardware sales. For ultra portable devices, in which category I would put many laptop lines, power and size are all that really matter. Granted, netbooks suck, but a device that's "good enough" speed-wise is fine for such requirements.
