A beaker full of death wrote:
The thing is, you can't destroy your hero's life every week. Consequently, most of the episodes were about the guest characters. The regulars were our entre into a world were anything was possible. It has the same principle as an anthology series.
We got to know the characters of Star Trek because we saw them in action. We got to experience things WITH them, not about them.
Good point -- that seems to be the '60s drama show mode. But character development is certainly possible in an anthology series w/o destroying anybody's life. Little moments or realizations or "B" plots. Witness TNG or DS9, which I bet we could agree had way more character development than TOS. They had 4 more years, of course, but still there was a different way of doing stories by the time the later series aired.