a) The outbursts, this by itself probably wouldn't have gotten him fired.

b) he tried to quit at the end of S1, so from producers standpoint he wasn't committed. Even though his performance though out the show spoke otherwise. The only way they could get him to commit to Season 02 was offering him a chance to direct an episode.

c) these guys (Crawford and Wayans (https://www.reddit.com/r/television/comments/j0czym/lethal_weapon_season_3/g6ssrnc/?utm_source=reddit&utm_medium=web2x&context=3)) hated each other and when it become one or the other the name recognition of Damon Wayans gave Damon Wayans the edge, but probably only slightly.