if you dont know touhou, you know ff. if you dont know ff, you know touhou. so creating crossover is very easy.
and both of them have many interesting factors.(characters, music, spellcard system, party system(touhous never make party!),etc...) so very interesting RPGgame was made.
then, why doesnt it become popular? as "crossover", it is very gorgeous. well, but I (and fans) know, to be modest is virtue. so, it is not tagged "touhou",on nico and pixiv. sorry for my long comment.
(tips, zun is doubted as a burontist(very uncertain), Tasogare frontier is surely burontist.)