|(this is not Kristy, it is just a cool Sax)|
God chooses His Prophets the same way I would choose a reed. He puts them to the test as they grow up and throughout their lives. Sometimes the individual is too soft, and when trials and affliction come around the give way to temptation, and brake. Sometimes they are too hard, or hard-headed, and refuse to yield to the promptings from the Spirit, and don't do as God commands.
But every now and then, when you find the right reed with the right thickness, you can play for 4 or 5 months without getting a new reed. With a Prophet, only those who follow the Lord and keep His commandments are chosen and ordained unto this calling.
"But Alma said unto him: Thou hast had signs enough; will ye tempt your God? Will ye say, Show unto me a sign, when ye have the testimony of all these thy brethren, and also all the holy prophets? The scriptures are laid before thee, yea, and all things denote there is a God; yea, even the earth, and all things that are upon the face of it, yea, and its motion, yea, and also all the planets which move in their regular form do witness that there is a Supreme Creator."