Because compliance - Riot won't take fan-submitted material and go through the effort of making it official due to legal reasons. Kind of the same reason why any other company wouldn't take a random person's logo of their company and make it official. This is the kind of stuff that requires clear and defined legal boundaries.
Also there are massive lore threads where Riot discussed certain champions with the playerbase. To date, nothing from those threads has graced the actual lore of the game in spite of the participation. As a result people are (rightfully) bitter, since in spite of player efforts and community engagement Riot still has nothing to show for it after years of promising that they would. Again.