Probably because he couldn't balance the kyubbi and senjutsu chakra together unlike Naruto was just able to do.
The reason Naruto couldnt balance it at first is because the Kyuubi was blocking it. We can tell Minato's Kyuubi isnt acting an ass like Naruto's was because he went Fox sage mode easily.
or he just gave him senjutsu in a spur of the moment thing like it looks like kishi did
isnt all natural chackra the same? so how do people do different transformations with senjutsu or is it completely dependent on if you have a summoning contract
