For me it was a few things:

  1. Small size (easy to carry around)
  2. Keyboard that's even smaller, super light, and optional (easy to carry around)
  3. Fantastic pen (better usability)
  4. Full OS Capability (better usability)

So, they focused on portability and usability, and did really well with both. And when compared with Apple tablets, they have better non-trivial usability, and when compared with Apple laptops, they have better portability.