It's not. But because of stupid government regulations that don't keep pace with technology, this is what we're left with if we want small electric vehicles that are street legal.
Two and three wheeled street vehicles are regulated by laws governing motorcycles. Anything with four must adhere to much stricter safety standards and expensive crash tests.
Would you believe people have even had problems with getting inspection stickers for electric cars in some places because they can't be tested for emissions?
You'd be better to ask why they didn't go with three wheels instead of four. In that case, I suppose that there may be regulations which forced their hand with that as well, or perhaps they chose to go with two because parralel parking becomes a breeze since you can just pivot on the central axis.
Then again, maybe it's just because they're Segway, and that's what they do. They wouldn't make a three wheeled vehicle, because that's not a technology they can really market, improve upon, or specialize in.
Well you put dinner in your backpack and you hang your backpack from the conveniently located hook on the back and then you don't hit any bumps on the way home.