Not sure why they didn’t pick it up, but it wasn’t worth thinking about too much. Not until recently did I think about dropping a package into those mail bins when I had an Amazon return with pre-paid postage that wasn’t being picked up by my mail carrier (Amazon told me to put into my mailbox). Can I drop off my package in USPS mail box?
I also never thought that I could use them for anything outside of what can fit into an regular-sized envelopes. I’ve used these boxes periodically for decades and never had a problem. For the remaining 99.99999999999% of us, those blue boxes work quite well despite our complaints about the United States Postal Service.