Trump spent the last 2 months telling his base almost daily to not vote by mail, so they didn't. Coincidentally, voting by mail is easier and faster than going and standing in a line for 4 hours, so it drove higher turnout for those willing to vote that way. The "fraud" you think you see is those mail in ballots coming in (most states have to count in-person votes first; it's literally the rules) and being overwhelmingly Democratic because the Dems didn't spend September and October scaremongering over their legitimacy and because the ease of voting this way drove turnout to record numbers.
Trump fucked himself in the ass over mail in voting and is now bawwwwing that his anus is sore. He could have blown Biden out if he'd told his followers to vote by mail, too, but didn't and that's on him. I hate to break it to you, but for as good as he is at triggering blue Twitter checkmarks, he's kind of a dumb dude with little to no ability to make long term plans and this is proof positive that's the case.