I guess the only reason for the less availability of XSL-FO processors is due to the fact that it's formatting options are pretty large and quite complex too..
Since the amount of complexity involved, no one wants to sweat so much and make it open sourced. I heard there were quite good number of cool commercial processors available for XSL-FO. Even Apache FOP is a nice one to generate pdfs out of it.
Though i haven't used LuaTex, i found XSL-FO is a fun to work. Also it uses just XML to generate pdfs. If you ask me i will tell, it's simple yet powerful. And also i don't want it to die either.. :)
