Mostly so as to charge money for the optional 225. The 198 and the 225 cost Chrysler the same to dress, build, and install, and the fuel economy difference was negligible. When Federal exhaust emission and fuel economy type-approval testing grew more complicated and expensive and the increasingly strict emission limits started taking a bigger bite out of performance and driveability because the US auto industry wouldn't spend the money to do the job properly, the 198 turned from a cash-maker to a cash-coster, and so it was dropped after the 1974 model year.
(Interestingly, the 1970-'71 198 gave better driveability than the 225; the Carter BBS carburetor, used on the 198, worked better than the Holley 1920 they put on the 225 in those years)