I would say because it's the first to seamlessly offer the option of portable or console play out of the box.
I'm pretty sure all other handhelds that had the ability to play on a TV first needed a revision or add-on to allow it and most of those came a few years down the road.
Break it down.
What's the difference between SNES and GBA?
Or 3DS v GC/Wii?
Multiplayer (single system), motion control, big 'experiences' (which is dependent on power) and 32" output.
Don't Play Stationary 4 ever. Switch!