The reason FTF has to depend on a bookmark list or some sort of tag in the log is because FTF is individual to each person. Unless we were "co-FTF" on a geocache (a concept I don't recognize, but for the sake of argument, let's say it's legit and we were), your FTF list and my FTF list are going to be different. So, project-gc needs additional data to determine our FTFs.
Conversely, challenge caches are open to anyone who qualifies. So there is no need to build challenge caches off of bookmark lists -- it's in fact counterintuitive. Once the site has a master list of all challenge caches, everyone will have the opportunity to have them displayed in their profiles. And if you want to track the ones you qualify for but haven't found yet, you can write a note and they'll show up in your challenges tab. (Or if not, you can continue to track them in your private bookmark list.)