It's been looked at (my advisor in grad school was working on it) and it keeps running up against the problem of scaling -- 100 million transactions (800 million if you want to scale up to India) on a single day is going to be hard from a realization standpoint, even if it's theoretically doable.

There's also the interesting psychological side to it: while a blockchain system is in a theoretical sense more transparent than anything else, from a popular standpoint it's incredibly opaque compared to a county registrar counting two different stacks of paper ballots.