Considering that Superman was invented in 1933, and had always worn red BVDs on the outside since inception, it took Superman not 10 years, but 80 years to learn how to wear his underwear inside. Luckily for him, he is nearly immortal, so he can afford to be a little slow on the uptake. If I may, your question can be rewritten as: "Why did it take Superman 10 years to learn how to wear his underwear inside?"