Why didn't running more made me improve more when I was a young teen (11-12yo) ?

What is your question again?

4-5 miles 4 times is just about 16-20 miles per week. And races of 1 mile to 2 k are quite short. Of course a low amount of slow jogging doesn't prepare you much for fast short races.

It's because at 11, boys are all over the place in terms of development, and their speed at that age is as much about puberty as it is about training. A kid who's hit puberty, is growing quickly, and has lots of T and HGH in his system will benefit more from training than a kid who's still a few years away from puberty.

Your easy miles definitely helped you, then later you hit puberty just when you added intervals. Lucky you! You wouldn't have been prepared for the intervals without the mileage you had done previously.