It wasn't so much his subject matter,stories, pessimism, preaching,smugness,misogyny, or even Leftism,et al, as it was his entire artistic approach and execution of his artistic ideas:
-the deliberately murky ,fuzzy photography, the camera almost endlessly slow-zooming-in on actors ( Too many times, to the point of gratuitousness ),and nearly endlessly wandering all over a set ,in the attempt to give the viewer the sense of the "atmosphere" of a scene,
-the non-directing of the actors( I've heard and read,many times, that "they never like to admit it, but actors need direction" ),leaving even great thespians flailing about, under / over-acting, in the style of the worst type of stage acting,, or, improvising that looks like the worst nights at Second City or The Groundlings,all in attempts at more "realism" and "naturalism" ,all of which only lead many critics to write "_________________ gives one of their worst performances". In no other director's movies was that written about any actors' performances more than his.
- the attempts at special effects( Slow-motion mayhem,a la Penn,Peckinpah,and others who did that better, the attempt at simulating cartoons for "Popeye",the earthquake in "Short Cuts", for examples ),
-attempts at referencing other filmmakers for some moments( Fellini,Bergman,Ray,Huston, and others ) ,to show how hip he thought he was,( And ,given his lack of talent, poorly executing them every time ). To name but a few things.
The end result,of all those elements ,mixed together, were movies that just lay there, and were rarely,if ever, as riveting,as movies with the same type of stories done by other great movie makers ,whether they were the ones who existed before Altman ever made his biggest films, while during the era of his main years ( 1969 - 2006 ),or after. While trying to sit through even what are said to be his "best" films, I know that any of his contemporaries would have done a far better all-around job on them - Huston, Aldrich, Wyler, Nichols, Arthur Penn,Coppola,Ashby, Kubrick, George Roy Hill, Scorsese, Peckinpah, Ritt, Pakula, Jewison, Don Seigel, Donner, Lumet, up to the Coens,Payne,Fincher,et al.
Richard Schikel and the great John Simon are right.
If anything,his movies are proof,that even if you are able to come up with a style that is unique / "different" and instantly recognizable, it does not immediately mean you and your films are "Great".