I think the qualities of a cassette mentioned have clearly helped with the mixtape model. But I can't help but wonder if it wasn't also a product of that particular era.

It certainly depends on geographical zones, too, but I remember people burning audio cds for quite a while, and taking them on the go with portable players. This was quite widespread before portable mp3 players became common.

Hell, where I grew up, cassettes were still in regular use until the end of the 90s, and mixtapes had grown increasingly rare.