What makes Riot refuse a good suggestion?

I've seen (and made) some good gameplay improvement suggestions, but oftentimes Riot simply won't do them. For instance, changing Irelia's W to give her shields instead of life-on-hit, or upgrading Ashe's passive to be actually useful in late-game. Or reasons to change the Sona rework.

Some of them are very well-founded and popular, but Riot simply goes "sorry, nope" and ignores it. Kind of a low blow, if you ask me, when a good suggestion or concept simply gets denied.