It is not necessarily the actions of parade participants, it is the press reporting the events that focus on the guys in the skimpy attire. We see in the paper pics of guys in drag or speedos. The TV reports that Pride weekend has begun and exception thousands to attend. The whole time the monitor is displaying a pool party or dance with guys barely dressed. There may have only been 5 guys at the pool party of 1000, but those are the ones the press show.
It isn't the event as much as the perception created by the press.

Don´t even forgot the role of Hollywood and TV movies and series. There are many comedies, where brief swimwear is considered old-fashioned or just plain funny, or something what is weared only by old, fat Europeans or perverts. That is for me the main problem, not a few individuals at gay parades.

Speedos are utilitarian, they are minimal to allow you to swim faster and sunbathe showing more.
Big baggy board shorts are like the togas of Roman times, they say who you are while being useless for walking, running and swimming in. Walking, running and swimming in big baggy impractical shorts says a lot about you, you don't have to try to hard.