Aside from the policy differences: power is one of the most addicting substances on this planet. Lots of people on this earth break the rules/laws to get ahead. Whether that is stealing, fraud, or tipping the scales in your favor in an election.
The Democrats have been out of presidential power for 4 years after an upset in 2016. What could prevent them from remaining out of power for another four? Tipping the scales in your favour, in localities (cities like Philadelphia, Detroit, Atlanta) that are heavily in your political control.
People talk about 'rule of law' like it's an end-all-be-all as to how humans work. But I think many act by a simple 'might makes right'; if it's in your power to win, then WIN, even if the rules/laws say differently. If throwing out the rulebook means that you will do better, then the rulebook wasn't good to begin with.
It's a lesson in power. I saw a lad from South-Africa comment on how this was the first third-world election for the US, and they should get used to more of them. I tend to agree that this won't be the last time we see this quality of 'democracy' in the Western world.
RE: Voter fraud