Variety is reporting that they did not quit but were fired because the directing duo did not see eye to eye with Kennedy or Kasdan.
Kasdan wrote Solo into film icon status with ESB and TFA so Im guessing it was his way or the highway and Im fine with that.
They also say that Ron Howard and Kasdan are in the short list of directors that could take over. If you have any doubts about Kasdan just watch Silverado. Great buddy western that he wrote, produced, and directed.