It seems some of this started really getting bad about the time Trump called them out for being inefficient and slow, in other words a typical bureaucracy. I have a suspicion that they, from top to bottom, have been and are still throwing a "postal hissy fit", using what has become the standard excuses like: covid, the weather, Christmas rush, Amazon, increased business and you name the excuse to jam the works.
You'd think that with increased usage they would have increased operating revenue which in turn would allow them to keep up with the increased usage. However, I don't think they think that way. I believe they were perfectly happy plugging along in an old, lazy mail/package service, quasi governmental bureaucracy culture, periodically going to Uncle Sam for bail outs- kind of stuck in a marginal business model with a lowest common denominator mentality keeping the status quo....which appears to be their comfort zone.