Because Clarence jumped in the water before George could and this saved him.
He jumped off of the bridge into the river, screaming for help. That forced George to jump in and save him rather than jumping to commit suicide.
Clarence saved George Baileys life by jumping into the icy river after George and pretending he was drowning. George felt obligated to save Clarence's life, and in doing so saved his own.
his wings.Clarence was an angel in training. By successfully helping George he was to receive full angel status and his "wings"