When you are building a trilogy, it is not uncommon for the director of one film to collaborate with the directors of the other two parts. There are even some times when one director might have to ask the other directors to change something in their films so the film they are working on would not end up with a major plot hole. It looks like that is exactly what happened between Rian Johnson, the director of Star Wars: The Last Jedi and JJ Abrams, the director of Star Wars: The Force Awakens. It turns out that the last scene of The Force Awakens was actually in conflict with some of the planned plot points for The Last Jedi.

Here’s what Johnson told EW about the favor he had to ask JJ Abrams:
“Yes … The big [favor] was, I asked if R2 could come with Rey, and if BB-8 could stay behind with the Resistance,” Johnson says. “Originally it was BB-8 who went with Rey, which makes sense for the story in a way. But I asked, ‘Can you do me this solid and switch the droids?’”

When you look at the trailer for The Last Jedi, you can see BB-8 in action with Poe and Finn, so it makes sense that he couldn’t be the droid to go with Rey. That said, when you look at the terrain where Rey and Luke Skywalker meet up, it sure does seem like BB-8 would be able to get around a lot easier.

Source: EW