Since most orbits of KBOs are highly inclined, isn't it rather a "Kuiper cloud"? Most main belt objects are also more inclined than the eight recognized planets, but not as much as KBOs (and bodies beyond) which can orbit above the Sun's poles. Why isn't it called the Kuiper cloud then? Is it due to historical reasons (because the first discovered KBOs such as Pluto weren't as inclined as Eris for instance)?
Compiling my comments into an answer.
Kuiper belt is a "belt" because majority of the bodies reside in the ecliptic plane and relatively low inclination. There are handful of exceptions but these are negligible compared to tens and thousands of bodies.
Why Kuiper belt is a "belt"?
The rotational motion of the planetary disc from which the planets formed can be seen as resulting from conservation of angular momentum where any angular motion of the cloud from which the solar system formed is accentuated by the "spinning skater effect". When the mass gets closer to the rotation axis, the angular velocity increases. So, the bodies in the Kuiper Belt lie mostly in a flat disk forming a belt shape while the Oort Cloud assumes a shape of a giant spherical shell surrounding the rest of the solar system. For a detail explanation, please read userLTK's answer (and upvote is possible).
This created a prominent gap between Kuiper Belt and Oort Cloud both of which is connected through the theoretical Hills cloud. Please read: What's the reason for the hollow region in Oort cloud?