Commodore wanted new C64 drives to be backward compatible with VIC-20 and vice versa. They failed the second goal, and C64 sold ~10x the number of units VIC-20 did making whole exercise pointless.
All to sell more outdated garbage chips made by MOS instead of using proper FDC controller on CPU bus with cheap standard floppy.
In any case, I maintain the engineering wasn't at fault, having a CPU etc. Fastloaders showed it to be just poor software, and that's a point I did not make clear enough.