Minato knew that obito was a Space/time Ninjutsu user, He knew that Nothing will work on him except FTG, thus he never wasted his time going into sage mode.Maybe because he's letting people die just because he didn't feel it necessary. When your wife is dealt a death blow, and your son is nearly murdered by the man you believe to be the most powerful man in the world, you don't hold back.
At this point it's all speculation on why he didn't use it, Point is, it was obvious he was a sage.