They want JWs to look "business-like" all the time, 24/7.
Problem is, most JWs don't look business like even when dressed up for meetings/service/conventions. Most of them look exactly what they are... un-educated, ignorant, no sense of style window washers wearing cheap, off-the-rack, ill-fitting suits (disclaimer: not all JWs fall in this category, but it's been my experience that it was more often than not, including myself at one time)
"Therefore, we should give attention to our dress and grooming while in the convention city, including when we check in at the hotel. Arriving in shorts and a T-shirt would not reflect dignity ."
Addendum: "Sisters need to be aware that the length, color and style of their burqa is appropriate and that no exposure of flesh is visible. It would also be important to note that ornate embroidary and bright colors would also detract from the diginity of the convention.
Seriously though.... ever notice they stop short of setting in place an actual rule that you can't wear shorts/t-shirts/etc.? It's just a comment: "Arriving in shorts and a T-shirt would not reflect dignity", not a edict dictating that one cannot wear such items. But it does create the air of peer pressure to scare you into not wearing those items. They coereced you to do so without ever really telling you not to.
Of course, it only takes one or two, corn-cobb-up-the-ass elders to go around taking notes and names and then counseling anyone who didn't follow the "rule". And most dubs never realize that there was not hard and fast rule and cave into the pressure from the hardnose types that like to impose their will on everyone.