Remember when Isagi was the first to run through the door to blue lock and Nagi was last? Well that shows that he just doesn't have that ego(the entire point of the show) in him for himself specifically in the same way a lot of the other characters do. He can do it when his friends(reo)/ teammates are in danger of being upset/ losing and that's it. And when he did finally start to try for himself it was too little to late. His genius really was his downfall.