AFAIK 35mm cassettes have become expensive in production, too. I think I remember a comment in regard to that by the Adox CEO.
Much too low demand. And you need a different machine for 220 format. It cannot be produced on 120 machines. E.g. Ilford stopped production of 220 because their 220 machine was worn out / broken. And a refurbishment was much too expensive in relation to the extremely low demand. There was an explanation here on photrio about that by Simon Galley of Ilford.
The only commercial 220 I know of currently is Shanghai GP3, and reportedly it's largely a by-hand operation. A few people on here do hand-roll their own also.
I've shot one roll of the Shanghai in 220, and have four more out in the freezer. I just shot it last week and am stalling a bit on developing, partially because I just ended up with a couple of nice Nikor 220 reels but am waiting on delivery of a tank to hold them(they require a 4 1/2" tank, not the standard 3 1/2").
I'd love more 220 selection. Everything about it is less expensive to shoot. 220 backs/inserts for almost every SLR are half or a third of the cost of their 120 equivalent. When 220 film was regularly available, it was usually 1.5x the cost of the equivalent film in 120(the last Velvia 100 in 220 I bought was about $65/box when 120 was running a little over $40 a box then). The lab I use to use charged 1.5x their 120 prices for 220, although I've seen everything from the same price to double the price.
As a nice little treat too, I found out that my fairly new to me Pentax 645N gets 33 shots on a roll of 220 compared to 16 on a roll of 120. That was really nice to find.