Sundays

I would like to know why the USPS doesn't deliver on Sundays. Everything else is open. When I was little grocery stores were closed. You may find the random gas station open after church but before noon. Now, yesterday, We can buy wine on sunday and cant understand why a store wouldn't be open. Why doesn't the mail come? Don't they know I am waiting on important life altering mail? I still get my e-mail, I still get phone calls from telemarketers, I can even do banking in some stores and online. I think we pay too much for a stamp to not have delivery on Sunday.