Happens all the time, depending on what you mean by "hacked". But online banking isn't the thing that's the most like voting -- ATMs are. How often do ATMs get hacked? All the time.

The huge issue is anonymity. Everything there is to record about ATMs gets recorded, to go back and look at later if it turns out there is a problem. You can't do that with voting. It's a secret ballot, meaning that it should be impossible to determine who voted for whom. At the same time, it should be the case that every vote is counted as the voter intended it to be counted, that no one should be able to vote more then once, that nobody should be able to pretend to be somebody else (living or dead) in order to vote as them, vote in elections that they are not qualified to vote in (by not residing in the proper area), and that nobody should be prevented from voting when they are qualified to do so, by not having ID (many people don't), by not speaking the language, by being blind, deaf, or both.

Clearly, these issues, especially uniquely identifing people without ID, is hard, and by focusing on the issues specificly of poorly impelemented electronic voting systems, people are ignoring the larger issues that these electronic voting systems are supposed to be solving.