The root issue at play is the drive by several states to begin "All-Mail Voting". Instead of visiting polling places to vote, registered voters would be mailed ballots to be sent back.
From the article on All-Mail Voting on Ballotpedia
Five states – Colorado, Hawaii, Oregon, Utah, and Washington – conduct what are commonly referred to as all-mail ...
TL;DR: USPS is currently facing major financial issues due to an unsustainable model and struggles to work as it is. Additional load from mail-in ballots would definitely add to the problem.
First thing: how many voters can we expect?
According to PennState University:
Around 138 million Americans voted in the 2016 presidential election.
From Business ...