But Findlay Crawford actually drives BETTER after a few cocktails! It relaxes him!

Excellent question.

I would submit several reasons:

(1) Filming Locations: the early Columbos were predominantly filmed on locations, really cool locations. Ken's lake cabin in "Murder By the Book", Vivica Fox's farm in "Lovely But Lethal", the house and beach scene with Robert Conrad in "Exercise in Fatality" and on and on. (The Hollywood Bowl scenes in "Etude in Black" are another great example). Some of the scenes were obviously filmed on a Hollywood stage, but the majority were on location. The later Columbos, while including some on-site filming locations (Pepperdine University for a lot of "Columbo Goes to College", for example) seemed to be filmed quite a bit on sets and stages. You can almost here the director yelling "Cut" half a dozen times while they try to get it right. Filming at a real house or beach gives the episode a realistic movie feel and not some cheap made-for-TV straight to DVD piece of garbage. There's no real "authentic" atmosphere in many of the latter episodes due to this. They're dull because of where they were filmed. It feels like they took the cheapest way out in order to get them made, while in the original episodes, they spared no expense to get the right touch or "feel" for an episode.

(2) The use of music. In early Columbo episodes, Patrick Williams came up with some quite inventive scores. "Try and Catch Me", "How to Dial a Murder" and "The Conspirators" are wonderfully accented by the score. Each note adds suspense and mood to their respective episodes. The later episodes lack that musical spark and inspiration. Rather than use original scores, the producers resort to pre-recorded songs by artists ("Columbo Goes to College", "Columbo Cries Wolf" and "Columbo Likes the Nightlife" are three examples I can come up with off the top of my head. Although, I like each of these episodes, the music lacks originality.)

(3) Quality of guest stars. Going back to three episodes I mentioned earlier: "Try and Catch Me", "How to Dial a Murder" and "The Conspirators" look at the guests: Ruth Gordon, Nicol Williamson and Clive Revill. These are three extremely talented artists, who each give a memorable performance to their respective episodes. For comparison sake, let's look at three of the last episodes of the latter Columbo: "Strange Bedfellows", "Murder With too Many Notes" and "Columbo Likes the Nightlife". Of the three, I find only Matthew Rhys' performance worthy or stylistic. George Wendt is pathetic and Billy Connolly was hardly stellar. Other than Wendt starring in "Cheers" and Rhys showing some acting chops, the guest stars pale in acting comparison with their series predecessors.

(4) Columbo Caricature: As others have mentioned, he becomes a caricature of himself, lacking the sleuthing abilities which made him so enjoyable and interesting to watch in earlier episodes. The ridiculous scene at the auto dealer "A Bird in the Hand", where he scrapes the floor and yuks it up with the wooden salesman and the crowd cheers him on. It's almost like he knows he's performing as Columbo and is looking for a reaction. It's uninspired and painful to watch. Going to the dealer and using a car in a secluded area, scrambling under the car, (adding some interesting music) and having Columbo get up and start walking away before he notices the scrapes would have been a better way of filming it in order to "fit" the character. His clues are nuanced (the feather in "Troubled Waters", the flint in "A Deadly State of Mind" to name two), not ham handed.

FILMING locations is one I never thought of.
The original did do their share of backlog work- Susan Clark's house is often seen on UI shows.

But, LA COLOSSEUM ( GAME).....Ca coast (PORT) The Citadel ( DAWN) Santa Monica Pier ( IDENTITY)....the construction site ( BLUEPRINT) Skid Row ( NEGATIVE)
are all interesting locations-