Why is USPS employee rated satisfaction so much lower than UPS and FedEx?

Does Employee Satisfaction at UPS and FedEx Affect Their Stock Prices?

From the article: "...FedEx employees gave their company an average "3.6" rating for quality. UPS employees, in contrast, scored their company a 3.3, and U.S. Postal workers gave USPS a lowly 2.8...."

Article Reference: www.dailyfinance.com