According to the same reviewers who advocated for The Last Jedi, Solo's box office failure is because of "franchise fatigue". I'm reading excuses that range from blaming the release date being too close to Avengers and Deadpool, to blaming Memorial Weekend itself, to blaming too many Star Wars movies in general with their annual release dates.
Reviewers are embarrassed that they got TLJ wrong. So many people hated TLJ that it's destroying the franchise box office. I sincerely doubt there's anything Lucas Film can do to save Episode 9. I wouldn't go so far as to say that TLJ destroyed the mythology. However, they certainly destroyed enthusiasm for nostalgia used to promote every Disney Star Wars film. Would you guys agree? I know some of you sincerely liked TLJ, but for those who don't, I think nostalgia is no longer a convincing sales pitch. The only reason for a Solo movie is nostalgia, and a lot of people don't care. TLJ is most likely to blame for that.
If you read the comments section of Variety and other articles, or Youtube comments, you get the same criticisms over and over again. The word "disrespect" comes up a lot, whether it be disrespect for the source material (The OT) or disrespect toward fans of the OT. Prior to Solo's box office failure, you could argue that angry fans were a minority of disgruntled Alt Right trolls, but now? It's not a convincing argument.