It may be because the Post Office is using more contractors instead of employees to deliver our mail. If had to guess, if like many other contracting firms, they are not being paid benefits, so the incentive to for individuals delivering providing adequate service to customers with such an arrangement is not as good.
https://about.usps.com/suppliers/becoming/contract-delivery-service.htm
https://www.indeed.com/viewjob?jk=9e884d7cdcb08074&q=US+Mail+Delivery+Contract&l=20151&tk=1chf9tfnt18523p5&from=web&vjs=3