Do you mean why were their names so short? Presumably Morse, Lovejoy, Frost.etc are deemed to be catchy and easy to remember. One-word names have always worked as they sum up the programme as a whole? Or do you mean the stature of some actors? Not suprising really, if the average height of a man is about 5ft 8ins. TV can make people look taller and fatter.
If you're talking about height, I suppose it's because really tall people are too big to fit within the camera frame! I'm quite serious: in order to get a decent focus and picture, the people in it shouldn't usually be very tall. Notice that basketball matches on TV never focus too close - the players wouldn't fit on the screen!