It's not about not being resistance affiliated. It's about that not being what his character's story is about, like I think the movie makes it. To me, they could unfold a story where Finn starts off the story conflicted about if he wants to help or not, not outright running away, but thinking about it. He can be working with the resistance in TLJ, but his main story arc not be joining the resistance. After he wakes up, he can be conflicted and torn between wanting to leave and helping Poe and the others. He eventually agrees to help, and tells them that the information they need is in the stormtrooper training facility, which only Finn, among them, knows the location of, being a former stormtrooper. He and Poe, and/or Rose or some version of that character if you want to use her (personally I think it makes sense to build on the bond between and Finn and Poe moreso than introducing a new character for Finn to bond with), break into the facility, Finn having a solemn moment where he has to wear the stormtrooper armor, in that Finn sees children being brought in by the First Order for training, and Finn begins to feel a sense responsibility for them and tries to rescue them. Now, the conclusion of this Ep 8 story can either be that they get out and this drives and sense of what Finn wants to do with his life, or he fails, and feels enraged and drives himself to take vengeance on the first order. The first option I think allows more for an affiliation with the resistance, without that concept driving his character. I think the 3rd movie could build on this more.
I was suggesting that Finn has no more of a reason to care than he did at the end of TFA. TLJ is the movie that decided he really only cares about Rey, personally. Why would I buy that he really cares at the end of this movie, after less time with characters, than he had in TFA, if TLJ would have me believe that after TFA he still only really cares about Rey?
I disagree. I think everything from the Kylo quest, Rey's story, Poe's story, Finn's concept, was basically a soft reboot, from what I remember reading/hearing. Maybe not thematically or something. I think very little in TLJ is needed for those things. Basically, to me, the pertinent info is moreso that Kylo took over the first order and killed Snoke, Rose exists and bonded with Finn, and Luke died.It doesn't really. TLJ is the movie that decides the First Order are basically a near Empire strength in takeover of the galaxy. Yes, TFA has the new republic planets seemingly wiped out. But that doesn't have to mean the first order are the empire and resistance are the rebels, again, needlessly. TLJ could've had a first order that put all of their eggs into the starkiller base basket, be trying to grasp for control in a galaxy thrown into chaos by the new republic's destruction, and the resistance fighting against them in it, making this more a tug of war for the galaxy. TLJ didn't do that. It opted outright for the repetitive rebels vs empire concept again.