Why did Venusaur drop to ZU in this Gen? After an amazing string of performances in UU and RU, how come it's all the down in the lowest tier?
The simplest answer is because of usage-based tiering. It’s B+ on the PU viability rankings, but since it hasn’t reached the 4.52% threshold in PU, it’s down here in ZU.
I don’t have extensive knowledge about any other tier so this next bit is all speculation. But with Vileplume and Amoongus lingering around NU/PU this gen (the sleep ban dropped Amoongus from OU/UU territory), Venu faces more competition from them as a defensive grass/poison.
Offensively, the only thing impressive for Venu is the fact that it gets earth power for coverage. 100 SpA and 80 speed isn’t a lot at all, especially for higher tier standards. It’s good in sun with Chlorophyll, growth (I think), solar beam, and weather ball, but sun is the type of strategy that tends to either be banned from a tier or too inconsistent to rise up its users out of low tiers.
Now, it’s INCREDIBLE in ZU. it’s bulky, strong, and/or fast (base 80 OP) depending on how you run it. It can run almost any EVs and can run any of acid spray, sludge bomb, giga drain, energy ball, leaf storm, sunny day weather ball solar beam for itself, earth power/quake, synthesis, and knock off. The power level is low enough in ZU for it to basically do whatever it wants. As you go higher up in tiers though, the power level gets higher, venu can get away with less, and it has to deal with more competition from mons like vileplume and Amoongus.