Nowadays we face terrorism as one of the most dangerous attack to our Democracies. Since in Democracy the political power is given to politicians by means of elections, a good goal for terrorists could be the alteration of electoral processes. If they could delegitimate the ruling power, they would have a great victory against democracies!
Ballot paper elections are very robust and have no single point of failure: there is NOT a single place which abnormal functioning could lead to the impossibility to declare the winner. Paper elections can be held despite of black outs and interruptions of computer networks. Infact paper elections have properly worked also when electricity and computer did not even exist!
On the contrary, electronic elections are based on computer networks and computer centers which are very good targets for terrorists. A terrorist attack to the network infrastucture, to power distribution lines, or to a computer center could lead to the impossibility to know who is the winner of the election. It would deny governments any democratic legitimation, seriously harming any Democracy