Chris Rock summed up successful black comedians in one of his stand-up specials. He said that black men have to be twice as good at their jobs as white men, and if he was a funny as David Spade, he'd be poor.
Anyway, I think the reasons Lorne and the rest of SNL's production team don't cast more minority performers are (1) they don't think they need more than 1 or 2 black performers (currently Kenan Thompson) and maybe one or two racially mixed performers (currently Fred Armisen) at any given time, and (2) Lorne's casting decisions have been notoriously terrible for the past decade and a half. I don't think they could find talented minority performers if they tried.
All race aside, up until a few years ago, MADtv was snatching up all of the real talent. Between Aries Spears and Frank Caliendo, MADtv had the better celebrity impersonators. All SNL has had is Darrell Hammond, although he is great at it. MADtv was recruiting Michael MacDonald when SNL was getting Horatio Sanz.
But you know what the really bad thing about SNL is? The shows long-running staples are the primary things holding it back. There's a reason MADtv only did weekly celebrity hosts for a single year. When you need to grab some random "hot" celebrity every week, you run a very VERY large risk of getting someone who isn't that damn funny. All they need is to do is call in an athlete, singer, or irreverent starlet, and suddenly every single sketch that week is being held back by a non-comedian who isn't that funny, can't remember any of their lines, and simply cannot perform live. Not only that, but their need to perform every sketch live (yes, I know it's been their name since season 2) stops them from making sure everything is performed correctly.
They only pre-record one sketch each week (and from what I hear, they do it on Friday night or Saturday morning), although I think pre-recording a large number of well-written backup sketches would be good to throw into episodes that are running a bit short, so we don't end up with the post-Update lag of long-running, unfunny sketches that were written on Friday night after rehearsal.
...and I'm turning this into an SNL thread.
How 'bout that MADtv?