In the series finale of Lost, why didn't Ben enter the church with the rest of the group? It was clear he had already fulfilled his tasks with Hurley as his "No 2" when Hurley was in charge of the island. Was there any explained reason as to why Ben decided not to go in? In-or-out-of universe?
Because he didn't feel he deserved the life he was beginning to experience.
To better explain:
My interpretation to LOST's ending, particularity the flash sideways, relies on the concept of the Island's "Heart" (life, death, and rebirth) with death being a transitional plane between one life time and the next.
Christian Shephard: This is a place that you... that you all made together so that you could find one another. The most important part of your life was the time that you spent with these people on that island. That's why all of you are here. Nobody does it alone, Jack. You needed all of them, and they needed you.
Jack Shephard: For what?
Christian Shephard : To remember. And to... let go.
Jack Shephard: Kate... She said we were leaving.
Christian Shephard: Not leaving. No. Moving on.
I believe the flashsidesways allowed the characters to glimpse/briefly experience the upcoming 2004 of the next timeline, where one group of people (variable set) had to remember the past in order to "move on" to the next time line, and in which they didn't have to directly deal with the Island [again], marking a sign of "spiritual progress".
Even though before he died (epilogue) he went on to help Hugo & Walt, Ben could not accept the life he was experiencing, because what he had done to Danielle and Alex. We may presume like some of the other characters that also did not move on, that he would have to face the Island again in the next go around, in attempts to do better.
As Jacob said, "It only ends once, everything that happens before that just progress."
As long as there is an Island with it's heart in tact, humanity can continuously spiritually progress and be reincarnated through the Island.
From Executive Producers Damien Lindeloff & Carlton Cuse explaining that the characters were alive until any given one had died.
On whether the characters were actually dead the whole time (Since the time the plane crashed): "No, no, no. They were not dead the whole time," Cuse said definitively, adding that he believes that some footage they showed at the very end of the series lead to much of the misunderstanding among fans.
"At the end of the series finale, [an ABC exec] thought it would be good to have a buffer between when you have the end of the show and when they cut to say, a Clorox commercial," Cuse explained. "We didn't have a lot of extra footage lying around, but we had footage of the plane wreckage on the beach," which they shot when the plane needed to be moved or it would have been washed out to sea. "We thought, let's put those shots at the end of the show and it will be a little buffer and lull. And when people saw the footage of the plane with no survivors, it exacerbated the problem."
But the characters definitely survived the plane crash and really were on a very real island. At the very end of the series, though? Yep, they were all dead when they met up in heaven for the final "church" scene.
On what the finale was all about: Carlton explained: "Very early on we had decided that even though Lost is a show about people on the island, really, metaphorically, it was about people who were lost and searching for meaning and purpose in their lives. And because of that, we felt the ending really had to be spiritual, and one that talks about destiny. We would have long discourses about the nature of the show, for many years, and we decided it needed to mean something to us and our belief system and the characters and how all of us are here to lift each other up in our lives." (Hence, the whole brilliang "Constant" concept. Swoon x infinity, Desmond and Penny.)
Damon added: "For us, one of the ongoing conversations with the audience and there was a very early perception, was that the island was purgatory and we were always out there saying 'It's not purgatory, this is real, we're not going to Sixth Sense you.' And we felt it too that the show had to become sort of meta in this way. And so the writers said, 'Obviously, there are all these mysteries. But what if we answered a mystery that was never asked, what's the meaning of life and what happens when you die?'"
Damon added that the idea for the "Flash Sideways" world came about between the planning of seasons four and five because "We were out of flashbacks and we were done with flash forwards. So we started to think about, what if we sort of Trojan horsed in a paradoxical sideways storyline?"