It's because it is transparent and can't be altered. That's the reason why blockchain has a better integrity if you're for that and projects that are into it aren't just for crypto but also for main use cases like voting.
A blockchain-based architecture does not have "better integrity" than other approaches per se--just ask one of the thousands of people who have lost
billions of dollars on failed blockchain projects. Blockchain can fail just like any other computer system can fail.
And no country is actually using blockchain for voting, which would be extremely stupid if they even tried it.