Simple, he used it in combat, didn't find it effective, ditch it.
Pretty much this in a way. But I think it's more to do with Minato's fighting style, his fights last seconds and he probably felt the opponents he was facing weren't worth the effort.
Of course he would find sage mode effective, that's isn't the issue because he's a perfect sage. It's just outweighing the benefits against the drawbacks. Increased Speed, Strength, Power but time wasted. Or less time wasted but less power (which could drag out fights more if he can't harm opponent).
It all depends on the situation, Minato didn't give a crap about the drawbacks when he insta went Sage Mode on Madara because he's the freaking Sage of 6 paths now. If he was facing someone like Hidan, what need would sage mode be.
His FTG kunai's are what kills platoons in an instant, Sage Mode isn't neeeded for this style of fighting. Only against kage level oppoenents and only the highest tiers of kage would he really need it.