Maybe because "purity" isn't all that important? I mean, when was the last time you heard a crowd pour out of a movie raving about the purity of its high-concept? ("You'll laugh! You'll cry! You'll admire how cerebral it is!") And, aside from its debt to "The Changeling," I'm not sure that TMP was more like a typical TOS episode than the other other TOS movies. It all depends on which TOS episodes you want to compare it to. I don't recall Kirk and Co. soberly encountering a pure scifi concept in every episode. I also remember them clashing with Klingons and Romulans, being captured by insane computers, dabbling in time-travel once or twice a a season, succumbing to strange alien viruses, dealing with interplanetary politics, bumping into Abraham Lincoln and Apollo, and even being overrun with tribbles . . . . I'm not sure there is such a thing as a "pure" TOS episode. Anymore than "The Changeling" is more like TOS than "Balance of Terror" or "Amok Time."