From a strictly law enforcement point of view, he is popular because he wanted his agents to be on the field and work cases. While he was himself a bureaucrat and not a field agent, he fought against the bureaucratic culture of the administration by implementing the famous "Hoover Rule" which states that an FBI agent must spend 90% of his day on the field and only 10% in the office.

He also focused on bank robbers and political activists which got a lot of good attention from the public.

He didn't focus on the Mob but it didn't impact him negatively at the time because people didn't know the actual power of the mob and the Mafia wouldn't affect them too much.

Interestingly, all the main FBI agents who actually worked undercover in the Mob (Pistone, Garcia, Ruskin) are all praising Hoover because he understood the importance on being on the field .