Is there any legal/historical/technical reason for this disparity?
America developed the Internet as a joint project between US military and key academic and large businesses. It was not originally envisaged to become a worldwide tool to connect everyone and everything.
As you may be aware, an IP(v4) address has 4 sets of numbers. Large organisations could initially get class A addresses, which are where they on IP.x.x.x There were about 126 of them in existence. (The idea is that these could be further broken down in the organisation).
A few orgs having lots of space, can skew things very badly.
(I note that IP addressing mechanisms changed, new blocks became smaller and more flexible in how they were routed).
Thus it is primarily for historic reasons and technical limitations of the initial design and purpose.