Not randomly, a Gorosei position was vacant and it needed to be filled by a strong character, Ryokugyu both worships the Gorosei and is strong, so why was Garling picked over him? Seems to be that old Garling > Ryokugyu at the very least.
Buddy the reasons do not matter, all that matters is that a character can become a Celestial Dragon if the Gorosei so choose it.