Your question is easily answered by the Pokedex entries of Uxie, Mesprit, and Azelf.
"When Uxie flew, people gained the ability to solve problems. It was the birth of knowledge." Uxie is the Knowledge Pokemon because it gave humans the ability to solve problems.
"When Mesprit flew, people learned the joy and sadness of living. It was the birth of emotions." Mesprit is the Emotion Pokemon because it gave humans the ability to feel emotions. (Or, at least, happiness and sadness.)
"When Azelf flew, people gained the determination to do things. It was the birth of willpower." Azelf is the Willpower Pokemon because it gave humans determination. Also, just because Azelf is the Willpower Pokemon doesn't mean that it should be a strong Pokemon. Willpower is a mental thing, unlike strength, which is physical.