I don't understand why a bank's website is secure enough to transfer hundreds of thousands of dollars, but I can't cast a single vote online.
The reward provides a sort of ceiling on how much time and money can be spent on trying to break a system. Even if you could get access to a few billion dollars by somehow "hacking" a bank and get away with it, that means that you'd only be spending up to a few billion to do it, and you may spend a few years to plan, but not much more.
But trying to "hack an election"? I would be shocked if there weren't multiple countries spending billions of dollars across literally decades of time trying to find the best way to influence and hack elections in other countries.
Do you really think that ANY software is secure enough to withstand a team of a few thousand of some of the most talented developers money can buy working full time for decades on trying to break it? Because that's the kind of threats that elections face. And if that election happens at all on the internet, the adversaries can be literally anywhere in the world and still attack it.
There are also auxiliary reasons why electronic voting isn't ideal. It removes the ability for the average person to validate the security of their election themselves without a software engineering degree (in a paper election anyone can go watch the locked ballot box all day to ensure there is no tampering, and they can count along with everyone else at the end of the day). It introduces accessibility issues, it adds unnecessary bottlenecks and dependencies (there have been countless stories of there not being enough voting machines or them breaking down causing lines to get multiple hours long and in many cases prevent people from being able to vote before polls close. There's even evidence this has been done on purpose to sway elections by preventing people in some areas from voting as much as people in others), and it's really expensive!
And what are the benefits to electronic voting? slightly faster counting? (you can get results in minutes instead of hours...)
It's possible that a much lower price would even work too, since if you were going to vote my way anyway, you'd do it for free. So I only have to convince the voters on the other side (and hope the electoral college doesn't override my proxy votes).
see 18 U.S. Code § 597: https://www.law.cornell.edu/uscode/text/18/597
Not only do you need every voter you buy the vote from to never speak up about it, but you also need to somehow hide all the mailing of absentee ballots, hide where you mail them back from, and your money needs to be clean and untraceable to yourself. AND all of that needs to happen right around election time.
But hacking an electronic machine can be done by a few people making changes to a warehouse of voting machines by impersonating a janitor 2 years before the election. And that's not hypothetical, 100% of voting machines that have been in the hands of hackers have been exploited to change votes. I myself used to sneak into the room where they kept the voting machines in my high school when I was younger.
"online" voting is even easier to hack. You could attack the users by buying a botnet and have it submit or change the votes on the client machines before they cast them, you could attack the servers via things like evil maid attacks on the server hardware, "normal" exploits on their software to gain access to the systems, even just paying the sysadmin that maintains it a few hundred million dollars to look the other way for a few days.
With electronic vote, both reward and verification can be done in untraceable way. Basically you could sell your credentials for Monero on some Onion site and vote will be done on your behalf. Low risk for both parties and plausable deniability for seller (OMG, I was hacked!!!).
A bank has no way to know in real time if you are being forced to transfer hundred of thousands of dollars or not.
USA is ruled by laws, not people. The laws can serve as a veil between those who set them and those who follow them. Online voting enables the SkyNet plot: laws are voted for online, but it's AI who casts them.