To put this in short-hand: "We bank online, we buy all sorts of stuff online, why not vote?"

The biggest reason is that banking and other financial transactions have a very different threat model from voting.

In particular, voting requires a secret ballot. In addition to preventing an adversary from learning how you voted, a secret ballot requires you to be unable to prove how you voted, to prevent vote selling and coercion.

So, unlike financial transactions, how you do validation / remediation of failures is very unclear. Ben Adida has a blog post with further thoughts here (https://benlog.com/2007/03/02/on-voting-banking-and-bad-anal...).