Genesis was able to have an SMS mode because SMS was a pretty clean design in the first place, based on tried and true interfaces and methods <s>stolen from</s> inspired by Texas Instruments chips, which were then expanded for Genesis. This isn't true of NES, which has heavily custom chip designs that interact with each other and the cartridge in rather odd ways. To add "NES modes" to SNES's processors would likely have been more work than just creating an "NES chip".