To be fair, Ba'ku is quite isolated in the middle of the barely navigable Briar Patch, and outside Federation territory, at least in the mid-2370s. It wasn't stated outright, but I got the impression that Nepenthe was a Federation colony, and was probably more accessible for the Rikers, especially with that spaceport they mentioned.
Plus Riker said that simply the soil was regenerative. The Ba'ku planet also has a ring system from which the regenerative radiation of that world came and Nepenthe doesn't have a ring system.