The first Fender bases had a 7.25" radius just like the guitars did. But I have no idea why they changed. Maybe they used the same radiusing machinery in the factory for bass and guitar necks? The 7.25" guitar radius had issues with low action and string bending, so they moved to the 9.5" radius instead which was an improvement. Obviously not really an issue with bass necks.

But unless they trialled it and people preferred the flatter radius on basses as well, it's probably all down to something pragmatic.

Or maybe Leo cutting costs again as they could use a slightly thinner fingerboard top for the necks with the shallower radius and get more blanks out of the same piece of wood?