Won’t work (for a quite long foreseeable future). Anyone can create an arbitrary number of DIDs. A DID is basically just a public key registered according to some standard (on a blockchain or otherwise).
You could either say that you don’t mean standard DIDs, but some proprietary variant where isssuing a DID is permissioned, controlled by a centralised entity, or you mean a DID combined with some verifiable credential about the holder of the DID.
But on the one hand, proving human identities to some “DID” or VC issuer will for quite some time exclude a lot of people who do not have passports or ID cards or whose country is not yet supported by those issuers. All issuers in that strange “self-sovereign identity” space just use classic KYC procedures to do that up to now, and those rely on people having government identities and on the issuer having the expertise to verify such a government identity for the country of the user.
On the other hand, ensuring that such a “DID” or DID plus VC is unique (which you want for voting applications) is hard. Either we rely on one issuer for the whole world or on a network of issuers, but in any case they would need a central database of all people who want to vote in Cardano governance.
And there is no obvious set of attributes that uniquely identifies a human being. “Jane D. Smith, née Miller” might very well have documents proving her identity as “Jane D. Smith”, “Jane Smith”, “Jane D. Miller”, and “Jane Miller”. On the other hand, you might have more than one “Satō Hiroshi, born 1988-08-08 in Tokyo” and you won’t want to say to one of them “Nah, you are already registered for voting.” because another one already has. And this is all still made worse if you – for privacy reasons – don’t want to store, e.g., names and dates and places of birth in a database, but only a hash of them. But such hashes are constructed to be totally different for small variations – leaving out a middle name, slight spelling variations, document with a date of birth only up to month or off by one day, old or alternative name for place of birth, …
Nation states can alleviate these problems by either having reliable enough registries of their citizens (like most European countries) or at least having the possibility to cross check their voter registrations and believably threaten attempted frauds with relevant penalties. They should be mostly okay (contrary to the beliefs of some US nutjobs). Rebuilding something similar independent of nation states, world-wide, on a blockchain, with users crazy opposed to anything that smells like centralisation or an authority is nigh impossible.
Worldcoin tries it with biometric markers. Apart from the unbelievably dystopian vibe of their strange orbs, this would also rely on a mind-boggling logistics to distribute those orbs so that everybody on the world can access one with a reasonable (for them) amount of effort and on a centralised control structure to ensure that all these orbs are built to the same specifications and untampered with.
On the third hand, as long as Cardano is quite niche and only a tiny fraction of the world will even be interested in participating, you can probably quite easily pay a few hundred people a couple of bucks to use their totally valid and totally unique identity to vote in Cardano governance.