B/c the design of the physical notebook made them have to choose what would work best for ~80% or more people. They choose 2 USB vs 1 USB and 1 FireWire. I don't think apple sees FireWire as dead in the bigger picture, they just don't see it as that important to include on they low end macbooks over USB which makes perfect sense to me.
Because the design is wrong, as with many Apple products.