From what I've read, Ron Howard and Kathleen Kennedy are pretty good friends who have worked together in the past. Which could put him in a position to take on the project. Likewise, Joe Johnston has the history with Lucasfilm and Star Wars. Looking at their IMDB pages it seems like both of them have projects in development, but nothing that's currently in production. That could mean that they might be available to take over the Han Solo film.
I had also thought "Why not bring in Gareth Edwards"? But as this story has been unfolding, I've also been learning that when it came to the significant reshoots that were undertaken for ROGUE ONE, Lucasfilm brought in Tony Gilroy to take over that end of the project from Edwards. And while I think many people found ROGUE ONE to be a great take on Star Wars, perhaps it wasn't the greatest experience for its director.