Why did they invent the nano sim card?

Smaller devices, and especially phones with dual sims.

Things they could have done with micro sims no problem.
My old phones had dual sim, used a micro sim cards, and were a hell of a lot smaller than my current, nano sim phone.

Agreed! I could see if the technology were radically different, but it is not. It is the same basic electronics. In fact, theoretically, you can cut a micro card and make it a nano..
A waste.

@LuckyGuy
That is what I did to put my old sim card into my new phone.

In fact, I bought a specialised sim card cutter just for that.

you can cut a micro card and make it a nano

This suggests they were probably conceived together, to accommodate both immediate needs, and future tiny-device needs.

Or maybe they were conceived together, to provide an easily handled card and a less convenient teeny card which fits teeny devices.