Without meaning to sound mean, that may be why Sophie was a good choice. Because if there were conversation (which surely S would ruthlessly keep to small talk) even if spun to the maximum, would probably not make a interview or book-worthy vignette.
Because - as I said in previous post - Sophie's work for some of her most important causes rarely gets coverage. We're more likely to see coverage of her with the military (where she is popular now) than we are to read about her speeches against FMG. So I'd be mildly surprised if the wider US audience even had a clue how she fitted in, and probably wouldn't care much what she did/didn't say on a car ride.
(As an aside - an entirely speculative one - I think it's quite likely that Meghan did not realise the existing broad reach of issues the RF was already active on, and may have thought there were gaps on important themes, such as VAWG, FGM and rape as a tool of war, which were ripe to be filled. But would have been steered away from, because of existing activity, which might not make the headlines here but was gaining traction internationally)