Probably due to whatever gained traction in the past.
WSP was developed on the east coast, so I still find it interesting that it's so popular over there.
Here are some of my ideal sized frames:
This is a comparison of full depth and ideal depth frame end bars:
This is a WSP frame being used for brood:
Nice thing about WSP is you can still run it for brood (as shown above in one of our hives). Ideal is getting a little shallow... but in saying that i've seen it done.. and pretty sure in areas of Tassie they run ideals for brood.