What is it about shoukaku that people picture her to be like this?

It's canon. Look at her damaged pose and curry contest episode.

rom_collector said:

It's canon. Look at her damaged pose and curry contest episode.

and to a lesser extent that curry competition

DAMN, Shoukaku-nee. If that's your regular wear, what's your "game underwear" like?!

NWSiaCB said:

DAMN, Shoukaku-nee. If that's your regular wear, what's your "game underwear" like?!

Maybe for Shoukaku, it's always game day.

zgryphon said:

Maybe for Shoukaku, it's always game day.

Or it might even be more risque. Like no-pan.
And have to say. This one is one of the rare ones where the unsuitable clothes doesn't really have anything to do with winter. That's a nice thing.