Your voice cracks because your vocal cords are developing. When they're not fully mature, it becomes difficult to control them, and your voice cracks as a result. Our voices can't change overnight simply because your vocal cords take years to reach full maturity. Don't worry though, my voice stopped cracking after a short while. Mine started cracking 8th grade, and stopped through half of 10th grade.
PhD student - Organic Chemistry
I don't read an entire thread before I respond; I try to answer the question quickly and concisely.