The only reason I could see them turning down a cybergoth/punk panel over steampunk might be that the person deciding had the connection in their mind that cybergoth = rave, and a few of the cons I've got to lately have been trying to convince con-goers to stop calling the dance a rave... due to raves being seen as something parents don't want their kids to attend. Not fair at all, but that's my guess.
The first couple years I was attending cons (back in the day~), yea, I had a similar opinion the OP had. Over the years though, I came to realize that if we snubbed out all the non-anime cosplay, there's a lot of fun stuff we'd never see. Heck, in the 11 years I've been going to anime cons, I probably remember more of the really good non-anime costumes than the anime ones. Heck, the only costumes I can specifically remember from my first Otakon was a group of Star Wars Episode 1 cosplayers. I don't even like Star Wars, but I was amazed they even had a kid for Anakin (it was the year Episode 1 came out).
I don't care what the genre a costume is coming from. If it looks neat, I'm going to take a picture of it.
Currently working on:
Nene (Bubblegum Crisis)