Because whomever owns the server is now a dictator who can decide the result of the election. When it comes to democracy, "tampering from the inside" is your pr

Because whomever owns the server is now a dictator who can decide the result of the election. When it comes to democracy, "tampering from the inside" is your primary threat model. Take it for someone who lived in a country where we had 70 years of elections with the same party winning each and every one. Tampering from the inside, when possible and hard to detect, becomes ubiquitous. We only started moving towards semi-fair elections when the election authorities begun to include representatives of all major parties. But in the server example, you would need very complex procedures to replicate that, and make sure there is not a single administrator that can tamper invisibly with the server. It is actually easier to build a tamper evident distributed system than a tamper evident single-node server.