For an election, you don’t need it to break down all the time, you just need it to break down during the peak of the election. And things on the internet do break down all the time, we’ve just gotten used to it. Today, for example, there are reported outages for Google Fiber, Cox and Buckeye. Sure, most of them don’t last that long, and most things people can work around. And as for data breaches, we’ve been getting sent new credit cards on a regular basis not because our card was stolen, but because it was included in yet another tens-of-millions-of-records breach at some retailer or payments processor.
Sure, you could in theory engineer a voting system that was resilient to all that kind of stuff at the 0.01% level (which is what you need to keep errors under 10,000 votes). But in practice it ain’t going to happen any time soon. And paper is really so much easier to make work from where we are now.