The question isn't about what Trump can do to stop it. David covers that well, though a lot of things we thought Trump couldn't do he is doing. It is about Trump's own explanation about why he can't. Trump claimed "I have an Article 2, where I have the right to do whatever I want as president". The Attorney General, well after the election, has issued a memo authorizing the Department of Justice to investigate voter fraud. He has pursued other expansions of executive authority and has pushed the bounds of Federalism. Why not for this issue?
I'll address Trump's own inconsistencies between his claimed problem, his claimed rules, and his actions. Because he did try to do something about it, or at least find his alleged fraud, back in 2017. We've been here before.
In May 2017, Trump established the Presidential Advisory Commission on Election Integrity to back up his claim that he "won the popular vote if you deduct the millions of people who voted illegally". The commission was led by Vice President Pence and voter fraud enthusiast Kris Kobach; another politician who has long made claims of fraud, is in a position to find them, but never seems to.
This was searching for the more traditional claims of voter impersonation, fraudulent registration, and double voting. These had already been extensively investigated and found to be vanishingly small. Most are clerical errors or honest mistakes.
The commission quickly gained national attention by making broad requests for voter information from every state including names, addresses, party affiliations, birth dates, felony convictions, voting histories, the last four digits of voters' Social Security numbers. For many states handing out this information was illegal. Other states provided what voter information was already public. Others refused to comply, pointing at no evidence for voter fraud, fears this information could be used for voter suppression, and questions about the integrity, security, and transparency of the election commission.
Despite making many claims, and after numerous lawsuits, the panel disbanded in January 2018 without presenting any findings or evidence of widespread voter fraud. After that the investigation was transferred to the Department of Homeland Security. DHS declared Kobach was not working with them and destroyed the voter data gathered by the commission; they already had access to it. As far as I know, DHS has issued no findings of widespread voter fraud.
Rather than concluding there was no widespread voter fraud, the Trump administration continued to make the claim. They blamed the commission's lack of findings on the refusal of states to cooperate, and the numerous lawsuits.
As with Trump's new claims of vote-by-mail fraud, this commission had made their conclusions and was searching for evidence. But the search simply highlighted what was already known: voter fraud is vanishingly rare in the US. As such it was embarrassing, counter-productive to the utility of the claim, and shut it down.
Why claim voter fraud and then not do anything about it? Why repeatedly claim it exists and is widespread enough to change Federal elections when decades of investigation has revealed only a handful of actual instances? Because the claim alone is useful for voter suppression or for de-legitimizing an election you think you might lose.
Kris Kobach, the man Trump entrusted with his Election Integrity Commission, provides an example. Kobach has a long history of making unsubstantiated claims about widespread voter fraud. He's used those claims to push for strict voter registration and ID laws known to suppress turnout. As Kansas Secretary of State, he wielded the power to hunt out and prosecute voter fraud. Kobach only filed nine cases and obtained just six convictions; all six were older citizens who were unaware they had committed a crime.
In Fish v. Kobach, Kobach was required to show evidence of voter fraud used to justify the proof-of-citizenship requirements of the Kansas Secure and Fair Elections Act. He was unable to do so. It was found Kobach had made substantial obstacles to voter registration in violation of the National Voter Registration Act and the 14th Amendment.
Kobach's supposed attempts to combat non-existent voter fraud were themselves found to be voter suppression.
Trump will not explain why he is unable to stop it because he doesn't need to. He merely needs to keep repeating the claims and get others to amplify them. Claims of widespread voter fraud themselves are useful, or Trump believes they are, to influence voters and policy. When put under investigation, such as by a commission or in a court, the claims rapidly fall apart.
Trump will, however, make excuses for why he is unable to find or stop it. These are useful to provide plausible arguments to keep the claims alive despite a continuing lack of evidence. Most recently unfounded claims of fraud have been used in court to try and halt or influence the counting of ballots after election day, but once again when pressed in a court-of-law they could provide no evidence. Despite this, Trump continued to make claims of widespread voter fraud stating on Nov 5th, 2020: "If you count the legal votes, I easily win, if you count the illegal votes, they can try to steal the election from us."
UPDATE (Nov 10th): Trump's Attorney General has issued a memo authorizing federal prosecutors to pursue any "substantial allegations of voting and vote tabulation irregularities.". He certainly thinks there's something the Federal government can and could have done.
The memo came after days of the Trump administration failing to provide evidence for their claims, and Biden's lead has solidified. This adds evidence that the Trump administration is using claims of election fraud as a tool to remain in power and investigating the claims is a last resort.
Richard Pilger resigned as head of the Justice Department's election crimes branch in protest because the memo defies long standing DoJ policy of extreme caution in public investigation of an ongoing election. Such an investigation risks legitimizing the claims. There is no evidence justifying taking that risk.