Earlier quoted context omitted.
Can the PIN change? How to issue new key if needed? How does it integrate with the voting?
Voting, much like all other things in Estonia such as getting married/divorced, doing taxes, signing documents, starting/closing companies, notary dealings, bank dealings, selling/buying vehicles, and many more things I can't even think of right now are entirely done via the digital ID that every citizen has. This means that you authorize/sign actions with it, including voting, because only you have your private keys…
Phones and sim cards a lot more temporary than ID cards. I don't know of a lot of theves that target ID cards for their authorization uses. Phones... people will steal those.