Yeah the inversion of a whodunit sounds like it would be boring because you know at the start who the murderer is.

But in a regular whodunit they can't really focus on the murderer since they'll give away the game. And you can usually just figure it out anyway because there's usually a character that has no reason to be there... unless they're the murder. Then the reveal at the end it's like, yup that character is the murderer.

The Columbo inversion of this allows for the focus to be on the murderer which is far more interesting. And usually the murderer is a smug rich asshole so watching him pester the shit out of him until he makes a mistake is very satisfying.

It also avoided the trap of getting into the personal life of Columbo. It's just about catching the criminal, not soap opera bullshit. Sure he mentions he has a wife (and a cousin, and an uncle, etc) but you can't really be sure if that's all just made up by Columbo to hustle the murderer into giving something up.

Fun bit of trivia: What was Columbo's first name? They never said. With HD TV you can make out a name on his ID, but they never said his first name throughout the show's run. It was all about catching the murderer.