WARNING: Spoilers for Star Wars: The Last Jedi follow from this point on, so read at your own risk.
In the original Star Wars trilogy, Luke Skywalker frequently received advice from the Force Ghost of Obi-Wan Kenobi but he was nowhere to be found in The Last Jedi. However, in The Force Awakens, the Jedi Master's voice - a mixture of Alex Guinness and Ewan McGregor - could be heard telling Rey that she was taking her first steps to becoming a Jedi. So, why didn't we see or hear him in the sequel?
McGuinness obviously died back in 2000 and writer and director Rian Johnson explains that it simply wouldn't have made much sense for the Ewan McGregor version to show up as Obi-Wan Kenobi.
"Believe me, man. I would have loved to have had Ewan McGregor in the movie but it was just a matter of storytelling. The original relationship with Obi Wan — obviously if Alec Guinness were still with us that would have made sense. But 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 recreate that character [practically], so it made sense that Yoda be the one that comes back and kicks [Luke’s] butt a little."
McGregor could have appeared in old man make-up but it definitely would have been distracting and a CGI version of Guinness could have been a tad controversial based on the reception to Peter Cushing's Grand Moff Tarkin being brought back to life in Rogue One: A Star Wars Story. What do you think?