Not that I would know but I'd assume it's just one of those early internet things. Like somebody decided that 6 levels of headers were good enough hence h1 through h6, and well you need to be able to distinguish them by size but also h1 shouldn't be too big so h5 and h6 being smaller than body text is a good compromise?

It's not like we had decades of UI experience then, so it would surprise me if the thought process was much deeper than that.