Sounds like you got pretty comprehensive answers already but let me get my own troupe straight in my head in your thread here.. this is most definitely going to be TLDR..
So the most inside based Insider of all is our Core (not a designation, just a name, but you already know that). Core just doesn't belong Outside. He belongs Inside. He is not even really a he, he doesn't have gender or age. He just puts on a male body Inside like someone else puts on a pair of slippers. He started off as a rational voice, like a secondary conscience for each of Us. Really he just wanted to be a writer.. so he wrote the story of our life. I can only hear him occasionally now but his voice used to be my constant companion "she typed on her computer... she did this, she did that,.." a running commentary in the third peson past tense. ARGH! I don't know if he still keeps track like that quietly or if he's just branched out to something else. Maybe pushing us towards accepting each other's existence really took his all. All those years stacking the deck in our favour, no wonder he cried when Mara finally figured it out for herself. But that's what he has been doing all these years. Because though few of us could hear each other, most everybody hears Core. He wanted us to work together so he led us in a certain direction.. He just wanted to stay the feck inside.. and when we were unaware he'd have to jump to Front and save us from stupid situations we got ourselves into acting alone out of context. And he hates being in the darn body. It's just not his gig at all. So for Core, definitely a choice. And a very valid one.
Then there is Pony. Sometimes I feel her co-con but really she is an Insider. She is only little, she gets scared easily and a fantasy world is quite good enough for her taste.
Next up: Mal & Dez. They also spend an awful lot of time inside and often don't come out for ages. I know Dez just gets tired of Hosting for long stretches and Mal Hosting alone makes everybody tired because his emotional range hurts like a claw hammer in the stomach. Not always, sometimes it's awesome having him up Front. But sometimes it really is an onslaught. Boy feels what he feels when he feels it. Not that Dez doesn't, but somehow shiny happy flake is easier to stomach than furious nihilist.
So they do spend lots of time Out, but sometimes they don't come out for two or three or four months flat. I think. But then they have each other I guess so they're quite happy taking breaks from the world chilling at the summer palace. Although, to be fair, up until Feb it wasn't exactly chill inside. Wartorn more like, but they still had some safe places and they were both adept fighters, so they found beauty in the carnage..
Mara is out lots and lots, she gets practically stuck out Front and also she gets scared to let go (she fears she'll lose time HA HA HA TIMESHEET!!!) I don't know about Rae because you already know I usually feel kind of cut off when it comes to Rae. Or Rae-Rae as she seems to call herself actually.
So yeah.
I think excluding Core there is an aspect of safety in choosing to stay inside.
As our internal world and our relationship with each other becomes a little more stable people seem to choose to come out more and at more appropriate times maybe? Because it's starting to be less of a clawing for time or being triggered out by circumstances and more of a measured approach.. slowly. I guess we are learning to trust each other a little more.
But there is also the fact that for some Hosting seems to be really tiring and the emotional landscape that spills over from Other Hosts wears everybody else out. Like honestly, I love Mal to bits, but him out at work in a stinking mood kind of stays with you long after he disappeared back inside again..
That said, Parts being scared of letting the Others reign free definitely plays a big part in who comes out/stays in for us. Like, Mara doesn't get half as antsy now when Mal is out, doesn't try to claw control of the Body back. Still, everybody gets overprotective when Pony is close to the Front and if Lud is anywhere nearby everybody is wearing figurative full body armour.
It's a learing curve.
I think the more you get to know each other and the more you develop trust in each other, the more things will make sense.
Yoda. NOT.