Why do people think the election was rigged
The answer is mostly that people think the election was rigged because it easily could have been rigged. Edicts about the World-Historic safety of our mail-in voting notwithstanding, there has been little in the way of a clear description of how we make sure invalid ballots are not counted. And this should be easy to do. Indeed a secure election is self-evidently secure, made so by the safeguard of public observation which brings about common knowledge impossibly difficult to baselessly dispute.
The principal issue with voting by mail can be summarized in a word: intention. Intention is hard to counterfeit and intention leaves a paper trail; intention guards against intimidation and harassment; intention can be audited. The difficulty of casting many fraudulent ballots scales much more than linearly in the number of ballots cast with the difficulty of casting a ballot. We can deduce that fraud is immeasurably easier under trivialized mail-in voting as a result. This idea motivates the protections usually place for absentee ballots. 
Examples of this principle in the footnotes may interest, but what is relevant is that Democrat lawyers, operatives, and officials litigated out or outright and unlawfully flouted every relevant safeguard required by Law or Reason. Sometimes cynically so: Pennsylvania’s governor threatened to veto legislation barcoding paper ballots that you or I could easily print at home.
All I require to shut up is a simple explanation of the procedures that were in place that make it unduly difficult for a malign party to print, harvest or otherwise procure, submit, and then have counted a relevant number of ballots. Or rather is it that the people who have been psychiatrically obsessed about Literally Hitler wouldn’t dare stoop so low as to rig what is riggable?
Did all this make a difference? Give or take: there are about 40,000 “ballots” between the President and his second term in the official count across several states. Simply rejecting ballots at historically prevalent rates delivers several multiples of the remaining margin given partisan bias in voting method. Or consider that the number of ballots counted secretly (as a result, unlawfully) in the wee hours of the morning — in cities that have a history of fraud, in states whose election officials were ranting like lunatics that the Reichstag is burning — is about an order of magnitude greater than the President’s deficit.
Indeed some of us remember a lot of crazy stuff. Precincts that stopped reporting because printers ran out of ink. Flash drives with tens of thousands of votes (need you ask for whom?) that were lost then found then counted. Delays in counting ostensibly because pipes apparently broke in tabulation rooms. Large and extremely skewed updates (need you ask for whom?) reversed upon inquiry because of typos or suchlike. Then the counties across the country where computer “glitches” resulted in thousands of votes in each counted for Democrats that were cast for Republicans, flipping several races. And it goes without saying the glitches only went one way (need you ask for whom?) and the explanations given don’t make sense. None of these facts have been coherently tracked and are only documented as a series of disjoint comments in local newspapers drowned in endless factchecks that misrepresent the underlying claim or outright lie.
Anyway: the point of an election is both to win and convince everyone that you have won. Voting by Tweet for a justice of the peace in a free, open, and civilized place like Nova Scotia is one thing. But we do not live in a free, open, and civilized place. Our press is violently biased and all our media of communication are oppressively censored by a well-coordinated and censorious cabal of progressive blackguards. So this election was always going to invite unprecedented skepticism whatever the result. The only antidote to that would have been unimpeachable transparency in the process, which we did not have. The perpetrators of this obfuscation have to live with the consequences of apparent impropriety whatever the ultimate result of this sad affair.
FRANKLY what is the evidence for the baseless claim that this was a free, fair, and secure election?
 Postal stamps require intention to procure and were they required to submit a vote by mail, a third-party ballot counterfeiting operation would have had a harder time succeeding discreetly. You would either have had to counterfeit stamps or bought them in bulk, each in its own way a forensically vulnerable undertaking. Democrat lawyers successfully argued stamps were a poll tax.
 Requiring a proactive request, your own signature, and two witnesses to obtain and cast an absentee ballot requires intention. Without such, ballot harvesters can procure ballots on behalf of nursing home residents among others whom they may intimidate and on whose behalf they may sign easily. Democrat operatives successfully used the pandemic as an excuse to “expand the franchise” of the old and enfeebled with a loophole that did away with requests and allowed signatures by proxy.
 Same thing for signature verifications. The point of which is not (really) to stop an enthusiastic voter from stealing his brother’s ballot, but making it harder for third-parties to introduce into counting many apparently valid ballots because it is not easy getting thousands of independently valid signatures. Democrat lawyers argued against robust signature verification and Democrat officials instructed poll workers in Democrat cities to ignore errors altogether.
 Barcoding ballots demands intention across the system, mainly in the process of tabulating requests by requiring a unique corresponding entry for each ballot sent in the county’s ledger. An incompetent poll worker would have a harder time sending ballots where they do not belong. This also makes it much harder for a third party to submit counterfeit ballots. The Democrat governor of Pennsylvania refused the Legislature’s request that absentee ballots be barcoded.