The Lord Peter books were mysetries, first and foremost. They worked even before Harriet came on the scene (first person to mention Peter/Bunter will get such a whapping...)

Hart to Hart were (IMnsHO)romantic dramas, more than mysteries. Moonlighting and Remington Steele I'd say were more of a mystery, but I will damit it brushed the line more often than not. The thing was, the romance supposed to be an adjunct to what they were doing, which was solving the puzzle.

But keep in mind that you're mentioning the handful that worked, and worked well enough to be remembered. Out of how many attempted? You may be using the expecptions to prove the rule, here...

Or maybe not. It is, as I said, a theory.