Why is solo rank elo more important than ranked flex elo in the community?

Because on SoloQ it tests your solo abilities, by that I mean playing without knowing the other 4 persons on the game. Playing solo you will learn how to make it thru by yourself, playing the unknown. You give your maximun on the SoloQ because you don't know the abilities of your team either your enemies. Flex is different, you can get to challenger by just playing safe and being carried.