Why is heresh like shoteh and katan?
Answer:
In times of Chazal and later generations, somebody who was deaf (the reference in Chazal is to a deaf-mute, which is quite rare) was effectively cut off from the world around him, and his behavior therefore deteriorated, so that he was treated on the same level as a shoteh and katan. Indeed, until the 17th-18th centuries or so, deaf-mute people were often institutionalized in an institution for mentally retarded people.
Today, many authorities rule that a deaf person who can read, write, and speak sign language, will not be treated as a shoteh.
For a detailed analysis of the deaf-mute and his status in halachah, see here, and for an English article see here.
Best wishes.
