There was no room on the spool for the longer rolls in medium format unless the full-length backing paper was omitted, creating a product which required different means to produce.
Oh, really? For Bronica ETR series, KEH right now has a 220 Ei (Ei=latest) back in EX+ for $29, and a 120 Ei back in BGN for $69. They have a 220 Ei in BGN for $12.By now it's a chicken and egg problem: nobody would buy a 220 film back on the off chance that one gets the film of choice for it and without such a back nobody will buy 220 film. Contrary to was was written here, 220 backs are not offered that much cheaper than 120 backs, at least not on keh or ffordes.
A back is 120 or 220 depending on which insert is in it. A 220 Ei insert, EX+, is $8. A much older 120 insert, BGN, is $21.
I prefer 220, and am sorry to see it used so little.