Part of their "reasoning" is that Russellites have them, imitating C.T. Russell. That's a historical fact that many Witnesses, including heavies at NY, and even GB members themselves, may well not be unaware of.
Another factor is that they are stuck in the 1950s mentality when male members in audiences of American TV game shows and male fans in stadiums at professional baseball games wore suits & ties and women attending wore dresses.
Of course, they cannot see the illogic in their frequent statements about how one would show disrespect to Jehovah by not dressing up like those old audiences did.
It is often impossible to find genuine logic in their thinking—they are quite traditional, you know, the very thing that Jesus condemned Jews of his day for being. I remember one brother saying to me that if Jesus came today instead of the first century, he would be clean-shaven and wear a suit and tie. I was speechless...