I have been wondering about that for years. Why doesn't Hollywood/TV try to find anyone who commands the screen like Jack Lord, even as a “one off.” Instead we get Mcactors where anyone can fill the bill. The closest thing we get these days is roles like Sam Waterston in Law & Order acting high horse.
I think because back before cable a 25% share would be considered bad, so you needed the best actors, best writers, best directors, best everything to be a success. Today if you can get 10-20 million people to watch your show you have done your job.
This is why I watch cable tv shows where there is sill fierece competition for ratings. Breaking Bad, Monk, Burn Notice, Psych etc. All are much more enjoyable then the big three have made in decades.