120 and 220 are on the same base - at least for the last few films that were manufactured in both lengths.
The differences are:
1) the 220 film is twice the length of the 120. Fairly easy for the manufacturers to deal with, although the costs of setup and take-down of the 220 cutting regime are probably quite high per roll;
2) the edge printing on the 220 is different. Again, fairly easy for the manufacturers to deal with, although the costs of setup and take-down of the 220 edge-printing regime are probably quite high per roll;
3) the paper leaders and trailers for 220 are special purpose purchases, are very expensive per roll, and have extremely high minimum order requirements from the sole supplier. As they have different start marks and other markings, and are different lengths, and require two separate adhesive tapes, the film manufacturers cannot easily re-purpose 120 backing paper;
4) the machines that put together 120 film, backing paper and spools cannot be used to do the same for 220 film. There probably are two remaining, working 220 machines left in the world (Eastman Kodak and Fuji) that will do that, but they may or may not be still functioning. Ilford's machine was long ago worn out. They costed out repair or replacement, and it was totally uneconomic. Hand assembly might be possible, if customers were willing to accept much higher prices and lower quality;
5) the exterior packaging is different between the two sizes. Fairly easy for manufacturers to deal with, although the relatively small volumes probably mean higher costs; and
6) distribution related costs are slightly higher for two different products than for a single product. At the retail end, the retailer incurs higher inventory related costs for two different products than for one.