Why is poor grooming part of the diagnostic criteria for schizophrenia? is it different from poor grooming found in other disorders like depression?

Not necessarily: Impairment from a mood disorder or a psychotic disorder can prevent people from self -care. It is an indicator of function.

Poor grooming: Along with delusions, hallucinations, disorganized speech & behavior, and negative symptoms like affective flattening, part of the criteria for schizophrenia include social/occupational dysfunction. Self-care can be part of this, and lack of it can show in poor grooming. Depressed people may also have poor self-care and/or grooming, but their overall symptom picture differs from schizophrenia.