I know why no one use Clarity
: Wait, but that's my passive, I'm suppose to be uniqu...

: Wait, but that's my passive, I'm suppose to be uniqu...

No-one used Clarity even before Unholy Grail existed. It's always been bad, because mana alone isn't worth a summoner spell slot.
I'll have you know that I take clarity in ARAM....! sometimes...
Though I haven't bothered with it since chalice is currently bugged and giving 5x the mana it's supposed to lol.
I forget who said it but some Rioter basically put it bluntly
"It's nice to have extra mana but the cost is you lose lane"