I hope the Marco expalantion isn't all.
Because that would be stupid as well as Neko or Inu could have contacted the WBs easily and Ace was actually there
I could see some elaboration but it seems pretty simple to me.
Wano is a closed country, so no one knows what’s going on inside it, which kaido and orochi have greatly taken advantage of.
Oden deliberately didn’t want to get anyone else involved, he viewed it as his own mess which he had to clean up, which ended up being a terrible decision. Him even actively preventing WB or the Roger pirates from getting news about kaido wouldn’t surprise me.
It’s necessary for the plot. In theory he could’ve asked for both WB and Roger’s help, and kaido and his whole crew would’ve been history in less than an hour.
Ace was a random teenage pirate at the time. And if he begged WB to go save every set of oppressed citizens he came across, they’d be spending their entire lives on it.