Its because madara didn't try it.Obito made TSB out of BZ just because Obito still had the RSM.minato pointed out that.
So Black Zetsu's mass is the only known substance that can be used to physically create a TSB. Kinda makes sense, I guess. But Black Zetsu is sealed away, now. What's there for Naruto to make more TSBs out of?
Also, I would figure if Madara was as smart as he seemed, if he thought it was possible to just
make TSBs, he would've tried at least once. He had plenty of opportunities, and seeing as he only had one left by the time Naruto and Sasuke gave him a challenge, it's not like there was any reason he shouldn't, if it's as easy as just willing them into existence. Assuming Madara isn't just dumb, it's only logical to assume that creating TSBs isn't that easy.