sensitive subject.... indeed it is... because it's tied with traumatic childhood experience...
it could be several different reasons but feel free to correct me. IMO - for many deaf people's bad writing - I believe it is because of bad education in their youth. It could be because they had undiagnosed problem (learning disability, dyslexia, ADD, etc.) that was hampering their attempt to learn writing. or it could be because they had awful (or... abusive) teacher or parents that discouraged them from learning to write. or it could be because they were at orphan/foster/state house so they never had proper guidance.
That is what I said, except I left out the part about having undiagnosed problems. You just paraphrased my statements, so we are on the same page, lol.