Blimey, who's hairbrained idea was that!
http://en.wikipedia.org/wiki/Subscriber_identity_module
Going from the "full size sim" - YES, they really WERE like a credit card size, to the current standard "mini Sim" was clearly a no brainer, and the full size is a dead standard unless you're a retro mobile collector.
The justification for even the "Micro" standard is questionable, and the Nano is just an excercise in making things difficult, just like having a new nonstandard accessory/power connector at a time when everybody should be using micro-USB.