It takes a long time to detect and fix problems. That's OK with ATM machines, because if you catch the insider who tampered with them months or years later, you can put them in jail and probably get most of the money back. But reversing election results more than a day or two after the first announcement is really bad for the stability of the country.
In fact, people are still digging into whether voting machine fraud happened in some states in the 2016 election. Any result now is too late.
Also, the nature of hacks is that you can often detect that one occurred, but not exactly what was changed. How would you take the news, "It looks like the Russians had root on every voting machine. But we've reconstructed the correct vote counts from analyzing deleted database files found in the free block list, and the winner is..." Not too convincing.