Read other letters about this article
This may sound trite or simplistic, but simple aging may have a lot to do with this phenomenon. Very few artists are able to sustain a high level of output, no matter what their particular medium. I could name several contemporaries of these men (not comedians) whose star has decidely faded over the last 15 years. Scorsese, Coppola, DePalma -- none have produced work that approaches their very best when they were at their peak. Same thing in music: Springsteen, Elvis Costello, U2, etc. And, as Stephanie points out, comedy may be the most difficult form of all.
(I do not include Adam Sandler because he is relatively young and egregiously not funny.)