Why isn't Sejuani not used more?

I just bought her on a whim when I reached 4800IP and JESUS she is amazing in a Support role. My first 2 games were great.
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The dash/knock up
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The constant slows
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The AOE stun ult which also procs her E slow
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Can build tanky as hell yet still be able to do a moderate amount of damage
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High base mov speed
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High AOE damage potential and great wave clear
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Passive grants great survivability
How is it that she is not used more? What flaw am I missing here? I assume its an obvious one?

is better as a jungler and
is better as a support :D
atm.
that are ult reliant, until level 6, you're mostly an xp leech

can sometimes be referenced here)
then get
something i was very succesfull with if im not behind is
as a core build. This gives you absurd cheat like damage with respectable survival you do build tanky from there on out though