Why did Venusaur drop down to ZU in Gen 9?

Question

After it's great string of RU and UU performances, why did it just suddenly drop 3 tiers down?

Comments

I think the best response to this question has less to do with answering it and more to do with the context it's leaving out, like Venusaur's placement in VRs...
B in OU: https://www.smogon.com/forums/threads/3734134/
B- in UU: https://www.smogon.com/forums/threads/3735310/
C in RU: https://www.smogon.com/forums/threads/3732106/
C in NU: https://www.smogon.com/forums/threads/3738305/
B+ in PU: https://www.smogon.com/forums/threads/3741667/
...and the fact there are no Drought users below RU.

There are heaps of sub-archetype abusers like Venusaur who have a niche in OU, but never officially reach it because that's just the nature of sub-archetypes.

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So basically the reason Venusaur dropped is that it is decent on certain teams, but not splashable enough to land in the other tiers? Makes sense if I read this right.

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There's probably more to discuss in terms of why sun didn't end up in a higher tier in Gen 9, but yeah basically

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Venusaur ranks higher in gen 9 UU than gen 8 UU, so it's possible that Venusaur is in the gen 8 UU tier only because a bunch of ladder players liked using sun teams. You'll probably get a better answer by asking here:
https://www.smogon.com/forums/threads/simple-questions-simple-answers-thread.3710884/