Why arent all Champions unlocked in Aram?

Can someone explain to me why Riot still hasnt made all Champs available for Aram? Because currently you can only play the Champions you´ve unlocked. I don´t see the point behind this, it is the only gamemode where you just totally have fun, playing totally random stuff etc. Im a Support main who is currently level 53, i sometimes play Aram because i really like the idea behind this, but than getting Champs like Janna, Leona etc. which i already play alot in Normals / Ranked just sucks.

I´m not sure if im missing a key fact to why they dont allow all champs but in my thinking it´really poorly Designed.

[sg-ahri-3]