To add to your point, also with ballot system it's very hard to change results. You can by a few votes, though it's a drop in the water compared how much you can buy and how many vote. If you want to change the outcome of the vote - you need to have a lot of resources (look at Crimea's occupation by Russia). Where's with e-voting, there could be few north koreans who would be able to compromise e-voting system in the country, which is on the other side of the world, just for the kicks.