Being first is no excuse! Britain (and empire) used to use an older design, the current design was introduced in 1947.

India (and other places) still use the older standard, BS 546. That's also used for things like theatre lighting in the UK.

They needn't have been so bad: one small change--tilting the pins at an angle--and they could've been mechanically sound like Aussie ones.

NEMA 5-20 is exactly that. It's somewhat less common, due to the fact that it implies a 20 amp supply. Most plugs and sockets use the 15 amp version.

Aussie plugs are more like NEMA 11 plugs: the prongs are angled to form two sides of a triangle.

You mean NEMA 10?[0] All I can dig up for NEMA 11 are motors.

[0] https://en.wikipedia.org/wiki/NEMA_connector#NEMA_10

You are indeed correct.

I'm not sure the order these were conceived, but it may be that there was already a plug with a similar configuration.

http://www.jmjwebsitedesign.com/cdpro/wp-content/uploads/201...

That's some panoply of configurations! It's a pity the most common configuration is the worst of them.