I can't believe some of y'all are actually saying this. It doesn't make any sense. We know that Wano trades with outside forces like the underworld and the WG, do y'all legit think that all of those people in trade with Wano use the waterfall entrance to get into the country ^^". Or do you think it makes more sense for Orochi and Kaidou to have divulged the secret for scaling the waterfall instead.
This is simply plot convenience for Oda to take the Big Mom pirates of the picture for a little longer and nothing else imo.
Yes it is plot convenience to have BMP away at the moment.
But Kanjuro and Apoo each betrayed their groups (scabbards, alliance with Kid) to report to Orochi and Kaido... Big Mum tried to screw up Germa with a fake alliance, and Bege had a longterm plan to betray BM... Yet you think it makes more sense for Kaido to just openly offer BMP to ride the VIP elevator coz they suddenly decided to become allies? We have no clue yet what's their real purpose (it is supposed to be announced by Kaido soon), and that alliance could very well be momentary, like a truce... Once they reach their common target, they could (or not, but possibility is there) just go back to being enemies! So yes, from my point of view it is logical for Kaido not to tell BMP about the official entrance.
You mention the trade but, as far as we know if i'm correct, we only saw CP0 visiting Wano and we dont know if anyone else from UW or WG has actually came to Wano for trade, right....? And we also dunno how CP0 came to Wano, via the elevator or via the waterfall, it could be via the elevator but wearing masks or something so they dont know the way... Yes, it's purely hypothetical from my side, but dont forget Orochi was in a much better position to negotiate and request totally unreasonable conditions...
Now the waterfall, it can indeed cause several ships to break and sink, but the Sunny was safe, Law's submarine came without any problem, so it's not hard to think BMP could come up without suffering any damage.... ^^ BM and Kaido themselves would certainly think so (they already did once before King pushed them away), therefore they dont need to help them with that.
The only thing we don't know at the moment is IF they tried to go again during that week (probably not), and if not WHY... which i am sure Oda will explain