The inconvenient truths: Women are not as funny as men. The numbers tend to prove it.
Not many people want to watch a black guy except other black guys.
Comedic actresses are funny...Tina Fey, and Julia Louis-Dreyfus and a few others, but the vast majority of female stand-up comics (the type of comic best suited to the late night talk format) are just annoying, not funny...Wanda Sykes, Sarah Silverman, Joan Rivers, the Asian woman (can't even remember her name), etc.
Network executives care about one thing: ratings. Ratings drive revenues. If Les Moonves thought a black or a woman would draw the biggest ratings he would have hired one.
And then there's the audience: Predominately young white males.
So there ya have it. The explanation to the phony "lack of diversity" among late night talk show hosts.
I came across a standup comic on a channel the other night. I saw one routine and I found it a bit funny. I generally avoid all TV entertainment talk shows but this was the comedy channel I believe. What amused me was that I was once in the position of a former GF who asked me about one of those dopey Cosmo quizzes. The comedian said he was asked by his GF "Name a person alive or dead who you would love to have dinner with." He said he wasn't sure what to say so he just mentioned "Caligula" to get a reaction. After she yelled and excoriated him for not choosing her -- again, decades ago I was in a similar predicament and my mind wandered to great people of the past and even some that were current, the comedian concluded his routine with "Oh, of course honey it would be you...you. No one but you. I would want to dine with you...(then he paused) and you would be dead." I laughed.