Their egos as creative independent designers that think they can make a better design decision than simply implementing what literally everyone is asking for. The only time Riot ever implements exactly what the community asks for is when they've exhausted every other possible option.
Community: "Make ARAM actually random by unlocking all champs"
Riot: "What if we don't and instead just balance every champ in ARAM with damage/damage taken % buffs/nerfs?"
Community: "Make everyone get the same number of items"
Riot: "What if we just gave you compensation gold when you didn't get an item instead?"
Community: "Remove Akali's Obscurity"
Riot: "What if we literally change everything about her kit first?"
Riot 35+ patches later: "Okay fine we can't think of any other stupid way to balance her anymore so here you go, obscurity removed"
Community's Grandchildren: "Fucking thank you"