SPOILERS for Star Wars: The Last Jedi from this point on -- if you have not seen it, we suggest that you go and see it, then come back.
In an interview with The Playlist, the director spoke about the decision to include Yoda in the film, returning in the form of a Force ghost, and not Obi-Wan Kenobi. The moment in question, which is to speak with Luke Skywalker and give him a bit of an emotional and philosophical booster shot, seemed appropriate for either Jedi Master. So why not Kenobi?
“Obviously if Alec Guinness were still with us that would have made sense,” Johnson said, noting that he also would’ve enjoyed having Ewan McGregor reprise the role from the prequel series. However, he continued, “we never saw Luke ever interact with the Ewan version of Obi-Wan, so there’s less of the emotional connection and it might have been a little odd. So it made sense and we could re-create that character [practically], so it made sense that Yoda be the one that comes back and kicks [Luke’s] butt a little.”
Re-creating the character practically, as Johnson says, is a much more divisive point in the series that pure storytelling. Last year’s Rogue One not only de-aged Carrie Fisher for its Princess Leia, it also made a freaky-deaky General Moff Tarkin from a CGI Peter Cushing, who died in 1994. These choice were met with almost universal hatred and ripped audiences from the film's universe. If Johnson had taken this route with Guinness, who died in 2000, fans would just have one more thing to complain about in the controversial new film.