Because Mesmer didnt think his effects were due to suggestion. He thought he really did have special powers that affected his patients.
The process of Mesmerism involved high theatre, a bit like a modern stage hypnosis show. Mesmer dressed in a flamboyant style and there was a massive expectation that ‘something’ was going to happen..and it did.
Mesmer came unstuck when a rational scientific approach was applied to verifying his results. So called ‘magnetized’ water was seen to have the same effect as non magnetized water when the fluids were switched without the patients and Mesmer’s awareness, an early form of the double blind trial.
Modern hypnotherapists use the art of expectation to get results but hopefully by being honest with their clients about the limits of suggestion. Suggestion can have amazing effects on the mind and body but its not a cure-all.