I think because today's actors just don't have the charm they had back then.
It just never seems to work well with actors in the last 20 years.
Then again not everyone can be Michael Caine
Using this as a springboard to discuss Muppets themselves rather than their human guest stars: there is something to be said about how the original crew
were The Muppets who created the characters and performances out of their own minds, which created very consistent and naturalistic performances. This was how you could have so many interviews with Kermit and Miss Piggy where they felt just like real people. Jim and Frank
were these characters. When their understudies took over it was a change, but they'd also been doing the characters for years under the guidance of the originals. I never disliked Whitmire's Kermit and have always appreciated the way he portrayed Kermit as an aging figure. I don't think Muppets Tonight is any less magical than The Muppet Show despite all the years that had passed. The naturalistic dialog and performances by the cast gels very well with the human guest stars they had at the time who never felt like they were there for a quick bit. I will never forget Garth Brooks singing Fiddler on the Roof.
But what I'm getting at is that "The Muppets" used to be highly specialized actors and writers doing something that came very natural to them. It created very holistic and organic experiences. It was something where human stars could be inserted and not at all feel out of place among the puppets. But as we get further and further away from their peak, they feel more and more like objects being picked back up and less like living characters who have always been there. There is a difference between "The Muppets are back" and "they are doing more Muppets" to me, you know what I mean? A lot of the time modern guest stars have the same exact quality as if they're walking on a green screen. They're not
present with the Muppets because the naturalism as a whole has suffered.
I think the more recent Muppet content, starting with Muppets Most Wanted and continuing into The Muppets and Muppets Now, feels like Muppets being "done" instead of Muppets just "being." This can be a very difficult thing to describe. And I am hesitant to call it Nostalgia because the first Muppet content I ever watched was the 2010 film, which spurred me to consume a lot of nonconsecutive Muppets content. There is a certain magic that is missing right now.
I believe they can get that back but it takes
something they don't have right now. I wish I knew what it was.
Edit: Buncha typos.