Kelechi did have opportunities and with Gabriel Jesus signing in January and Kun's suspensions there were chances to show signs of improvement to the rest of his game (which does need more work). Pep helped to develop one of the best attacking players around so it isn't the case of not wanting to work with world class players
Some methods don't work with everyone and a different coach may be the thing for Nacho. Just because Pep improved Messi doesn't mean he can do that for everyone. Different approaches work wit different players. His opportunities withered away after one bad performance and pretty much killed him after that.