I would like to put it out there that his depression or whatever mental illness he had was on full display the whole time, and this probably resonated with people as well.

A couple years back I started re-watching all of his shows, start to finish, after watching Roadrunner. Especially the early seasons, there was rarely an episode he didn't joke about dying, being killed, or killing himself. (In the film, there was a quote from Tony about how an acquaintance observed they'd never met someone who wanted to die so much)

I think a lot of people picked up on that, and it made the whole the whole thing work. The grit, the machismo, the empathy for the plight of your fellow man. A lot of people who worked with him said he was an asshole, too. This is also not surprising that he would be at times when the cameras were off.