I just find the phrase 'The Motion Picture' somewhat pompous and needlessly wordy.
To be fair, I appreciate that it was a different era and the term 'motion picture' was more commonly used then than it is now. But...it just doesn't work for me.
Well, look at it this way: it'd be weird to call it ST:TM instead of TMP. Then it would sound like it meant "trademark" or something.