Actually, Victreebel actually has quite a better niche than Venusaur, do to the fact that Victreebel has access to weather ball, which is devastating in sun, and can run that along side HP Ice, giving it great coverage against the Dragon types that constantly wall Venusaur. Also, it has a great move in sucker punch that if the weather is changed and something faster comes in, it still has a way to deal with it. And the good thing about weather ball is that if the weather is changed, then Victreebel can have a fire type move, a water type move, an ice type move, and a rock type move all at the same time. Along with access to growth, sleep powder, swords dance, and a lot of the other moves that venusaur usually runs (bar earthquake), Victreebel has a niche to where it can be a more effective special sweeper or mixed sweeper than Venusaur can.
Also, the reason why Swift Swim is banned is because unlike Pokes with Chlorophyll, All the Pokes with Swift Swim are water type, which allows them to have +2 speed, and practically a super effective move on types that are neutral to water (with the 1.5x stab boost and the 1.5x rain boost), and even allows them to have practically an unstabbed neutral attack on types that resist it. On the other hand, all Chlorophyll users are Grass types, and the only Fire type move they have access to (bar victreebel with weather ball in sun) is HP Fire. It's also due to the fact that there are so many swift swim users available, while there are really only 3 viable chlorophyll sweepers (venusaur, victreebel, and sawsbuck.)