I've been following events in the PCUSA, particularly related to what marital/sexual standards are required of ministerial candidates (mostly through the GA Junkie blog). The amendment to remove restrictions on candidates has been defeated, but by a more narrow margin than similar actions in previous years (handy summary and comparison chart).
While this issue is of interest, I have to wonder: why doesn't the dissenting group just leave? Presbyterians have split over plenty of issues, why not this? The conservative position have generally been the majority and remains so with this vote. Why wouldn't the clearly sizable group in the minority that disagrees just leave?
I have perused the websites of both sides on this PCUSA issue to gain some perspective. One thing I noted from those that wished to remove the restriction: I tend to find them focused on the concept of equality, treating this as an issue of not accepting people equally. I also found that this vote was generally portrayed as a victory in that the balance of votes has shifted more to their side as opposed to previous efforts to overturn the restriction. Further, there was no sense of giving up - but rather that this issue will be challenged again and again through the workings of the church until they achieve the change they desire.
Let me add as an aside that when a church body is split 60-40 (or so) on some issue of significance, no matter what the issue, there is a major problem.
My reading of the minority groups (that wish to remove the restrictions) makes me assume that they are not interested at all in splitting, but their ultimate goal is reform of the PCUSA according to their design. My theory about this is that since this is framed as an equality issue, almost (or perhaps entirely) like a civil rights issue, that very much influences the desired goal. I would assume that the interest in that case is to have a church body that you're comfortable in and can believe and act as you see fit, but that could be achieved by a split. The more important goal is to reform any and every organizational structure that does not practice the desired equality. It would be unfair to make a comment like "well, why aren't they working on X much more conservative denomination?" becuase that's not where they are. I understand feeling the responsibility to bring about change where you are right now, rather than worrying (immediately) about elsewhere.
In light of the assumptions that the church has an equality issue (allowing the ordained to have various views and practices on sexuality outside marriage) and that it must be changed to enable that equality, it makes sense to keep pushing a particular issue until you achieve the desired result. Splitting is really not an option - you could only be forced out.
There certainly may be other reasons those in the minority stay, including but not necessarily limited to:
1) A high view of church unity. Perhaps this group desires to see the various calls to unity in Scripture truly lived out, even when serious divisions exist.
2) A love/commitment for the PCUSA. In this case, perhaps someone grew up in the church, has long been a part, and strongly wants to stay in the same institution (whether denominational or local church).
I'm not there and I really don't know anyone's heart - this is just a collection of theories. It is interesting (to me) to contrast this with the PCA which split from a body that eventually became the PCUSA. There, according to the histories I have read, the denomination leadership was pushing toward more liberal positions. In response, some worked against this, eventually decided they would not succeed, and split to form the PCA. In that case, I would assume that the purity of the church was the issue - that the mother church was increasingly not practicing and enforcing the marks of the true church (true preaching of the Word, right administration of the sacrements, and church discipline).
Given that perspective - that the purity of the church is to be pursued, a split is a more likely option. The conclusion that the mother church is too compromised to be brought back and that you're clearly a minority that will not succeed can pretty much only lead to a split.
I also wonder about the issue of submission to the authority of the church. When there is a controversy in a denomination and that denomination rules in a particular way, I do wonder why the minority would bring that issue up repeatedly. I would say that shows a lack of submission to the body. Even if you disagree and are in the minority, you should listen to the wisdom of the other brothers. Remember you're human and might be wrong. And if you're right, perhaps it is better to look to other denominations to join rather than continue to stir up a controversy when you are in the minority.