Why? Because it’s the left that commits voter fraud. Jumping to your assumption ignores this and your analogy is ridiculous. The Clintons in particular and the Dems in general have a long history of cheating going back to Tammany Hall. There is no history of stores colluding against Coke.
Trump didn't warn about voter fraud. He didn't tell the audience to get out there in large numbers to overcome fraud on the other side. He didn't tell them to stay vigilant and keep people honest. He said the election is rigged so he'll lose. Any candidate who says such a thing is a moron because he has instantly demoralized his supporters by effectively telling them not to bother voting.
... which is why he has never said anything like that again.