Ari Aster hasn't said anything about why Disappointment Blvd. was changed to Beau is Afraid. To speculate, it's likely that Disappointment Blvd. was always just a working title meant to obscure any guesses as to what the movie is going to be about. Even before production started and Disappointment Blvd. was the title given out by Aster and A24, people speculated that Aster's next movie after Midsommar would be an adaptation of Aster's 2011 short film titled Beau. With the release of the trailer and the knowledge that the actual title of the movie is Beau is Afraid, it makes sense that A24 would want to keep the real title hidden for as long as possible. If you've seen Beau (2011) and you've seen the trailer for Beau is Afraid (2023), you can see at least a few similarities. Beau is about a man on medication who has a strange relationship with his mother (who appears to be a monster), and who becomes increasingly afraid of people and the outside world. The trailer for Beau is Afraid shows clips of Joaquin Phoenix as a man on medication with a strange relationship with his mother, and who appears to be afraid of everyone. There are even other parallels including both Beaus wearing pajamas and cancelling a trip to see their mother. Obviously there is much more to the story in Beau is Afraid, but if you've seen Beau, then you might have a good idea about very minor spoilers for new movie begins. Alternatively, since people would understandably speculate about the movie based on the short, it's possible A24 didn't want people getting the wrong idea about the movie and they didn't want misinformation spreading too early.