... Why were only Democratic mail servers hacked...
Apart from, as Les said, practically asking for it? What exactly there was to hack from Republicans? Everything and a kitchen sink was already out in the open about Trump, 15-year old private tapes, etc. As I said before, perhaps only a sex tape (with a goat or a pre-teen boy) could have hurt him.
Did an average undecided voter even know who Podesta is? And if they did, did they actually know what he wrote in those emails? And if they did, did they actually understand what it meant?
Long before servers were hacked, people made up their mind. I knew a year before that Trump is going to win. Only (ideologically) blind didn't. To the last minute. The election was a repudiation of Democrats, not a vote for Trump. So strong a repudiation, that people even voted for Trump... Trump! Nobody loves Trump. The vote was against political correctness, against lawlessness, against creeping socialist totalitarianism, against identity politics, etc. People were so sick of that "fine china," that they were willing to let an elephant loose into the shop.