Popa – What “obvious tznius reasons” are there for disliking denim? And why do you presume to know better why Rav Falk dislikes denim than, say, someone who actually asked him (in this case, myself)? He associates denim with what miners and the suchlike wore in the 1930s, which was often denim due to its durability and toughness.
Unless I am reading your post wrong and you were being facetious. In which case, I apologise.