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 and SuSE endorsed it.
RHEL 7 uses XFS as its default filesystem, and it was an install-time option in RHEL 6. After RHEL 6 came out, Red Hat backported official XFS support to RHEL 5. XFS was available for RHEL 5 before that through the semi-official EPEL channel.
SuSE included XFS as an install-time option much earlier than Red Hat did, going back to SLES 8, released in 2002. It is not the current default, but it has been officially supported that whole time.
There are many other Linux distros, and RHEL and SuSE are not the most popular distros across the entire Linux space, but they are the big iron distros of choice. They're playing where the advantages of JFS and XFS matter most. These companies 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...