Why does'nt Donald Trump want us to vote by mail

Trump claims that police officers will be present at polling places which could cause some voters to stay home thereby suppressing the vote using fear, uncertainty and doubt.

Mail in voters cannot be suppressed in this way,

If the voting machines or software used to tabulate the totals is compromised they could be programmed to mis count or calculate totals.

There must be auditing and testing procedures used by voting officials to test their voting and tabulation systems. Hacking Democracy is a 2006 Emmy nominated documentary film. The film investigates the flawed integrity of electronic voting machines, particularly those made by Diebold Election Systems, exposing previously unknown backdoors in the Diebold trade secret computer software.

Paper balloting is more accurate than the other voting methods in use.

Having an accurate count may not be what Donald Trump is seeking.

Being non electronic paper ballots are not available on any computer network or internet so the ballots are inherently safe from hacking thru the internet. Proof that they are not hackable.

Electronic voting ballots could certainly be kept off-line to enjoy a similar safety to paper. It would be up to the implementors to enforce that and auditors to check compliance. But being electronic there is always a possibility that EVERYONE does not know what is really happening.