In a world where unimaginable amounts of money are moved around electronically every day, millions of online transactions are processed every minute, and I can pay my taxes, file returns, and renew my drivers license online - why is voting online “not safe” or insecure?
Each voter must be able to vote exactly once and their vote must be correctly recorded.
No one must be able to tell who voted for which candidate, so you can't be pressured to vote a particular way.
All sides involved must be able to verify that the first two things are true. You shouldn't need to trust anyone.
The problem with online voting is number 3. There's no way to make sure that your vote is accurately recorded without being able to trace your individual vote. With traditional voting, there are observers seeing with their own eyes that things are being done correctly. But you can't observe an online vote. Even if I show you a computer program that counts votes correctly, there's no way for me to prove that the computer program running on the server is the same program I showed you, or that there isn't some other program off to the side messing with the data.
The reason online banking works is because it doesn't care about being anonymous - every transaction you make is tied to your identity. You can see every transaction made in your name and verify them after the fact, even on the phone or in a brick and mortar bank if you don't trust the computer and you can contest any discrepancies. But that can't be allowed in voting.
Also; Attack vectors should have the least amount of impact. E.g. you got one district where there are malicious persons counting the votes. In paper voting this isn't even this easy because a lot of people have to keep quiet for this action. Even if this works you get a few hundred wrong counted votes. Online voting has a HUGE attack surface as there are so many parts you can attack - voting machines, voting software, file transfer, vote tabulation -and so many methods to do so.