Well, that is the conclusion of a study done earlier this year by University of Texas anthropologist Karl Petruso. His major trove of evidence? Well, faculty door postings. He concludes that history faculty could be–if evidence from one anomalous individual is ignored for the purposes of statistical rigor–”consigned to the depths of humorlessness.”
More on this can be seen here.
I attribute a lack of humor on the part of many academic historians to spending far too much time in archives and away from people! And, a high seriousness (much of which is affected anyway) that is adverse to relaxed humor.