That technique leaves quite a lot of evidence around.
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.