Does anyone know why.....

why would it be?

As it's been mentioned, this movie was a departure from what had been Carpenter's fare, predominantly horror and thrilling sci-fi (or whatever). It seems fair to assume that he didn't necessarily intend for this to be considered part of his general oeuvre. Not that it's a bad movie, just that it should not be taken with Halloween and They Live when thinking of his style and legacy.

also, i know my username may seem like a reference to this movie. it's not. it's my name.

I have no definite proof of this, and it's possible someone answered your question in this way already, but it seems to me John Carpenter is known, first and foremost, as a horror movie director, as Alfred Hitchcock was known primarily for horror movies (Psycho, the Birds, etc). And look at what happened with Hitch's last movie, "Alfred Hitchcock's Family Plot", a comedy with Bruce Dern and Karen Black, Alfred's only flop at the box office. People went, expecting "Psycho 2" and got crazyman Dern cracking jokes. The audience was confused, and so stayed away.

"Memoirs of an Invisible Man" is more an amalgam of a slew of preconceptions. People who went to see it when it was first released were expecting a "Fletch" like comedy, since Chevy Chase starred. Adding Carpenter's name to the title would have confused the movie-going public, thinking they were in for a gory terror flick, sorta like "Holloman" turned out.

Again, Im just surmising, but this makes the most sense to me. No director in the history of Hollywood has been more typecast than John Carpenter. Making "Halloween" for pennies and gaining back hundreds of millions, he doomed himself to make horror movies, by his own success at it. Because of it, people expect a certain product when they go see a Carpenter movie.

To some of the other questions in here, he was wearing his invisible suit (without jacket and tie) when the grocery delivery boy comes in and starts going through crap, in what I felt was a really contrived scene, put in just to give the audience a chance to sympathize with Nick and smile when he scares the kid into leaving. But to keep Nick looking sympathetic, the kid had to be a dickhead wannabe thief. Would the audience have found it as funny if Nick scared a Girl Scout delivering cookies?

Perhaps because people associate his movies with horror/thriller? If I were going to theaters to see John Carpenter's The Invisible Man. I'd expect something on the likes of Hollow Man. Although better of course. Besides wasn't this movie based on a book?

As previous poster stated, he was a director for hire on this movie and he only puts his name in front of movies that he had full control over.

from wikie

The director [John Carpenter] deviated from his usual practice of titling the film as "John Carpenter's" because he knew that Warner Brothers would not allow him full artistic control, saying that the studio "is in the business of making audience-friendly, non-challenging movies."

I first assumed it's because he didn't write the screenplay, but he didn't write VAMPIRES either and it's still John Carpenter's Vampires. So yeah, I guess the most plausible reason is that it's just not a "John Carpenter type film".
I don't mean to impose, but I am the Ocean.

it's because he thinks this light comedy film is beneath him, and he's right. this film truly sucked.