Because they just made it up
It does not track with the character....He literally refused to Kill Vader but Ben a kid..he's like no you die
Once again Luke in Mando, Battlefront 2 (he literally works with someone from the empire) and NEW books have Luke as a completely different character who doesn't judge others or make snap judgments even against "villains"
The director of Last Jedi wanted to make his film with his message doesn't matter if it fit the character or not.
I mean, Kylo literally flip flops the next film along with Husk, so yeah clearly Disney has no idea what it is doing sometimes
Sometimes writers like in this show have a message they want to push, doesn't matter if it makes sense the writer wanted to have the line about a religious group being unchecked.