I think the fundamental reason it's laid out like this is because it results in elements with the same major index being close to each other for caching. The a

I think the fundamental reason it's laid out like this is because it results in elements with the same major index being close to each other for caching. The assumption is that users will change their minor index much more often than the major one while accessing data, which means that cache prefetching will just work. Obviously there are situations where this breaks down, but if you have to pick one way to do it I think this makes more sense.