Who said they don't use it? Any statement must be based on facts and scientific evidence. New suggestions are built based on a person's past experience through a chain of associative links. If in real life experience a person knows and understands what it means: "Do not do", "Do not get sick", "Do not want to eat fatty and starchy foods", the suggestion will be correctly accepted by the subconscious of the hypnotist. The main thing is that this does not contradict the perception of the particle "not" by the hypnotist himself, and in the preliminary conversation this moment is spoken out by the hypnologist.
Those who work in the Ericksonian paradigm try not to apply in large quantities, but without fanaticism. In the Russian language there are such combinations that you cannot say otherwise than with "not" - more precisely, you can say it, but it will be pretentious - can interfere with the effect of the work. Moreover, in some cases, this particle is used specifically to bypass the client / patient's resistance.
Those who work in a directive format do not get stuck on the topic "not" at all, they have their own scheme.
It is believed that the "not" particle is not perceived by the subconscious. This makes sense - if you, for example, will instill in yourself "I do not want to smoke", then at the moment of pronouncing this phrase you will have exactly the image of smoking, which will only provoke the desire to smoke, if you have such an addiction. The fact is that the subconscious is concrete, and it cannot imagine the "emptiness" that this phrase implies. If you think that you want to be healthy, breathe clean air deeply, then this positive image is more likely to motivate you to give up the bad habit. There are other pitfalls - the phrase with the particle "not" is comprehended longer than the one formulated positively, and also focuses on the negative aspect.
But in fact, under certain conditions, you can use the "not" particle. For this, hypnosis must be sufficiently directive, and supported by additional suggestions showing the subconscious what should be instead of what the particle "not" indicates. It is also important to highlight the particle with intonation. When these conditions are met, the subconscious mind will understand the suggestion correctly.