The business logic here is >15 years old. Http compression was only in early stages then, and you can guarantee that many client side scripts and libraries would jot have supported it. Zip was well known. Compress and place.

If I'm not missing the point here, this was, and still is, about offering the simplest, most reliable solution over a long period. This is a near perfect example of how to do exactly that. No changing formats, no moving requirements, no big swings in frameworks, apis, or even standards. And most importantly, no breaking your customers business workflows.