Because the very idea of using document timestamping service to vote is stupid. Voting has different needs and thus cryptographers developed different protocols.
https://en.wikipedia.org/wiki/Electronic_voting#Cryptographi...
What computer-related fields call merely impedance mismatch between the tool and the job, in cryptography always ends up as catastrophic results. This is how it would be stupid. And add to that blindly following a fad (blockchain in this case), this is stupid, too.
> The main advantage I was seeing in blockchain is that it's immutable and distributed.
So would be DHT. No need to waste energy in calculating hashes.
And you don't need the voting system to be distributed, you just need it to produce results verifiable for everybody that took part in voting. If you had checked the link I provided, you would know that and if there are any systems that provide appropriate features.