The Hollywood Reporter has been gathering more details on what has been going on, but first and foremost, it should probably be stated that Lord and Miller didn't leave the movie, so much as they were removed from their roles by Lucasfilm Boss Kathleen Kennedy. They were dealing with "extreme scheduling constraints", a source says, and "were never given enough days for each scene from the very beginning". I dont know if its because there were two of them but they were not decisive. Production department heads began to complain. "You have to make decisions much earlier than what they're used to", one of these sources say.
In May, Lucasfilm replaced editor Chris Dickens (Macbeth) with Oscar-winner Pietro Scalia (The Martian). This includes hiring an acting coach for lead star Alden Ehrenreich - who plays Han Solo - firing an editor, and trying to figure out who to place the blame on. Tony Gilroy was brought in to polish the script for the reshoots, taking over as Gareth Edwards remained on as director, with Gilroy shadowing him in post-production. However, Disney CEO Bob Iger attempted to assure fans that the Han Solo film will be fine in the capable hands of Ron Howard during a candid exchange with TMZ. The performance is even being compared to Jim Carrey's Ace Ventura.
Whether this was more to do with Ehrenreich's inability to excel under Lord and Miller's direction and improvisational filmmaking - compared to his fellow cast members, at least - or not is unclear at this stage, but as THR stresses: "Hiring a coach is not unusual; hiring one that late in production is". The pair constantly ignored Kathleen Kennedy's guidance, as well as that of legendary screenwriter Lawrence Kasdan (The Empire Strikes Back). "The next day, when the crew was told that Ron Howard would take over as director, sources say they broke into applause", reports THR. Not only did the directors clash with Lucasfilm, but they also didn't have the best relationship with each other, apparently. There was also eyebrow raising over the types of shots the directors favored. Also, Kennedy has more than enough experience working with Star Wars, she is the president of the company, after all.
While everything is still up into the air regarding the Han Solo movie, fans can only hope that Kennedy and Howard can create a Star Wars movie everyone enjoys.