That may be more due to the roles he's offered.
In most of his roles, he's either a mobster or a nut. Many years ago, I was coerced into seeing "Stanley & Iris" and it was a completely different sort of role. Another role that comes to mind was when he played the honest, bus driving father in "A Bronx Tale."
If you check with IMDB, you'll see that he's actually played a lot of different characters in many different roles, it's just that his most "popular" roles have been either mobsters or nuts. How about his "HVAC Terrorist" role in Terry Gilliam's "Brazil?"
Mark
Characters with different names but still the same character. His voice is the same - his facial expressions are very limited. To me, most of his characters are the same guy with different names. IMO.