Because the Balance and Update team make poor decisions. This is evident with the horrible and unnecessary changes they gave to Xin, Graves, Yasuo, and more. The biggest problem is that the new items and keynote masteries they have YET to balance is benefiting a specific group of champions and leaving others high and dry. Yi and Fiora is a great example of item changes that benefited their already toxic kit. Yi was manageable in Season Five honestly. Fiora has been a problem since her rework.
In regards to Morde, almost all of the community hate the rework done to him. It's not that they are upset that he received a rework . . . it's the fact that Riot gave him a rework and tried to bottleneck him into one lane . . . bot . . . . and as a juggernaut . . . which essentially changed EVERYTHING about him. Let's add in the fact that they relied so heavily on the dragon mechanic that we lose the Morde that some of us used to love.
Karma and Ryze are other examples of reworks they botched, tried to force another role on to us, and have no identity in this game.
The problem is that instead of trying to fix their mistakes and appeasing the community that love these champions, they spend eight months on Soraka, give unnecessary buffs to Xin and Graves, and choose to update Brand and Vel'Koz over other champions they know need just as much work or more.
The Riot Balance and Update team need a fire light under their butholes or need to be fired. We need more competent developers that get things out faster and don't wait three to four patches to nerf overpowered champions. We need more competent developers that can promise reworks for champions that don't span over the course of a year or three.