The first thing you have to get out of the way is the comparison to ext. Replacing any of them is going to be like replacing NTFS in Windows. Possible, sure, but it will require a decision from the top to switch.
I know you're asking about keeping existing alternatives, not removal of other alternatives, but that privileged competition is sucking up most of the oxygen in the room. Until you get rid of the competition, marginal alternatives are going to have an exceptionally hard time getting any attention.
Since ext aren't going away, JFS and its ilk are at a serious disadvantage from the start.
(This phenomenon is called the Tyranny of the Default.)
The second thing is that both JFS and XFS were contributed to Linux at about the same time, and they pretty much solve the same problems. Kernel geeks can argue about fine points between the two, but the fact is that those who have run into one of ext's limitations had two roughly equivalent solutions in XFS and JFS.
So why did XFS win? I'm not sure, but here are some observations:
Red Hat endorsed XFS. Their official kernels have XFS enabled in later RHEL5 releases, and it's fullly suported in all RHEL6 releases. I seem to recall it being available as an experimental feature even earlier.
There are many other Linux distros, some more popular than RHEL in some areas, but RHEL is still the go-to player for running Linux on big iron, where the advantages of JFS or XFS matter most. Red Hat can't always wag the dog, but in questions involving big iron, they sometimes can.
XFS is from SGI, a company that is essentially gone now. Before they died, they formally gave over any rights they had in XFS so the Linux folk felt comfortable including it in the kernel.
IBM has also given over enough rights to JFS to make the Linux kernel maintainers comfortable, but we can't forget that they're an active, multibillion dollar company with thousands of patents. If IBM ever decided that their support of Linux no longer aligned with its interests, well, it could get ugly.
Sure, someone probably owns SGI's IP rights now and could make a fuss, but it probably wouldn't turn out any worse than the SCO debacle. IBM might even weigh in and help squash such a troll, since their interests do currently include supporting Linux.
The point being, XFS just feels more "free" to a lot of folk. It's less likely to pose some future IP problem. One of the problems with our current IP system is that copyright is tied to company lifetime, and companies don't usually die. Well, SGI did. That makes people feel better about treating SGI's contribution of XFS like that of any individual's contribution.
In any system involving network effects where you have two roughly equivalent alternatives — JFS and XFS in this case — you almost never get a 50/50 market share split.
Here, the network effects are training, compatibility, feature availability... These effects push the balance further and further toward the option that gained that early victory. Witness Windows vs. OS X, Linux vs. all-other-*ix, Ethernet vs. Token Ring...