Well, perhaps it has to do with listing things from the top to bottom. Just a thought.

It is hard for little kids to learn to count, conversely, the lines on the staff from bottom to top--or moving up. The first line, as you know, J., is the bottom line of the staff--and then you move up to the top line, which is the fifth.

Anyway, I'm not sure about down through the ages and all that, but maybe it simply has to do with how we list things on a page from first down to last.

If someone knows the correct answer to your question, I'll be interested in learning, too.