In this case it's extremely obvious the reason Luke wasn't there was because if Luke is there then the show is about Luke. They wrote themselves into a corner and they wanted Grogu back with Mando before the episode was out and they couldn't have Luke so this is what they came up with.
Unless this is a storyline going forward where they acknowledge Luke being an ass for sending Grogu alone, I refuse to even think about it from a lore perspective, it's clearly the writers just being stuck.
There was no need to make the reunion having place on a planet that was in the middle of a mob war. It cut down the emotional impact of Mando and Grogu being back together by having Mando being busy fighting and it made Luke an asshole.
They could have done it properly in episode 1 of season 3 of Mandalorian instead of this.