Ooh! I'm glad someone is taking the "why is modern spinning and medieval spinning completely different" question seriously.
As for the why, I suspect it has an awful lot to do with this: the craft revival in the 1970s was initiated and driven by people in the USA (and many/most literature has come from Americans and/or people taught by Americans (or people taught by people who were taught by Americans, etc.)). If living in the USA and trying to revive crafts, the principle source/inspiration was native peoples of the USA and central and southern America. They spin very similarly to modern spinning.
As for the why of that, perhaps it has something to do with fibres? Native American people principally spinning cotton and other shorter-fibred things, whilst Europeans historically spun linen and long-fibred sheep wool?
That's my best guess.