You would have to ask the original publisher why he chose it! To speculate:
1. "Time' on the cover of a magazine is very emphatic, as in 'Time is Money', (and the magazine is orientated towards how day to day world affairs affect economies), 'Time Waits for No Man', "Time is of the essence', 'Time will tell' It's almost anthropomorphic, personifying Time as some looming figure dominating our lives, and moving forward relentlessly.
2. 'times' on its own is associated in people's minds much more with low-key situations - learning our 2 times 2 tables; how many times do I have to tell you not to leave your toys out; oh, we've had some good times together. You will note that newspapers make it more emphaatic and important as, The New York Times, and in England, The Times, not just Times.
3. 'Times magazine' doesn't come off the tongue nearly as well as the simple alliteration of, Time magazine.