Probably for a number of reasons, some of the same ones that upright vacuums seem to be more popular now. First, most people want scrubbing action on the shampooer, and with a canister version this requires a lot of added cost and complication with a motorized power nozzle, solution and suction hoses. Upright versions are easier to set up and store than a canister unit with separate, hose, wands and nozzle.
At that time, Bissell and Kenmore were probably the bigger players in the market of household machines and only had the drum style cleaners. Then Regina came out with their upright machine with attachments, and by the time other brands such as Hoover and Eureka introduced models, they went straight with uprights and didn't bother making a canister version other than perhaps small ones for spot cleaning. Other than Rainbow, I don't really recall any household canister cleaning machines than the first two I mentioned.