Err, they don't & won't. At least not those working towards more serious creative aims (and you need to start from the understanding that they are definitely not your old cheapskate 3rd-party-lens-gobbling-AdAm's - they understand that going up in format is always going to deliver better results than trying to extort fractional improvements from a smaller format film). FX has been off the market for longer than I've been around - and I know that a large chunk of your putative market would try it once, see that it's noticeably grainier than Delta 100 or 100TMX - and closing in on 400TMY-II - & return to their 4x5 (why do you think Intrepid and Chamonix have sold so many 4x5's?). That's the reality, they'll try a roll or two of a novelty film, but for anything serious, the speed/ grain relationship has to be much better. If you were serious about this, you would know that there is another once well-known slow-speed emulsion from a major European manufacturer (that could be manufactured by one of the successor companies) that would potentially offer a significant granularity improvement for the low speed (even if less sharp than 100 TMX or Delta 100) but which was withdrawn in the late 1990s due to low demand. And while we're here, why do you think Ilford has not invested into updating Pan-F to resolve anything beyond immediate batch-to-batch manufacturing issues?