An explanation for hearing persons
A common misconception about American Sign Language is that it is a simple translation of English. Native Deaf ASL users find it difficult to process written text because it’s actually a whole different language.
Signed languages have a grammatical and sentence order very different from spoken and written languages and can prove difficult for sign language users to translate effectively.
Consequently, an ASL user has many of the same challenges with reading and writing in English as others for whom English is a second language.
People who were born Deaf to hearing parents often suffered from language deprivation during their first five years, the critical period of language acquisition. For these individuals it’s even more challenging to successfully learn to read and write English.