Nonetheless, there is a physical paper record of every vote that can be counted by hand, the machine is just for a quick estimate of the count. There are only 2 points of trust in a mail election: the vote itself which could be compromised by blackmail or whatever (low risk because only one vote can be influenced at a time), and the scantron machine (low risk because the ballots can still be counted by hand to verify).
But if your vote is just a byte getting added to a database on a server from your personal computer over the internet, there are a hundred high-risk trust points. You have to trust the user, but also the website, the browser, the operating system, the network, the hardware, the server, the software running on the server and everybody who has ever worked on it, the counting software, the people operating the counting software, and a compromise at any single one of these points could change millions of votes without any possibility of detection.
In other words, in a scantron election, the legal vote is the paper, the electronics are just convenience. In an internet election, the legal vote is the ephemeral machine-generated bits on a wire, and that's where the problem comes from.