@alison Online systems are compromised all the time. Online bank fraud and tax fraud, and identity theft that enables them, are common problems. The difference is that, because neither banking nor tax payments are anonymous, it's generally possible to either insure or roll back fraudulent transactions.
One a vote is cast, on the other hand, it can't be traced back to the voter and corrected later if irregularities occurred. A compromise of an online voting system could cause irreversible harm.