One of the consequences of a wisely used ~~but small~~ budget. Simple sets and costumes, plus a LOT of creative lighting. All that's left are actors to fill that otherwise empty-feeling space on the stage.

Also, color TV was in its infancy, so networks used the medium to compete for viewer attention. It really looks like NBC made sure that every scene was as vibrant as possible. But now that I think about it, it also had to "read" well in B/W too.

Edit: @StillPaisleyCat@startrek.website corrected me here. TOS was indeed an expensive show, which upon rewatching (as I am today), becomes more evident the longer you look at it.