Probably because 1) you have to order 25-50 boxes at a time... 2) many found out about this and are using the boxes for other things than sending priority mail
....
I used to do a lot of flebay business and it was a piece of cake to have all my stuff ready to go before I got to the post office.
A funny thing is I had alot of left over boxes and tried to take them to three post offices to return, none would take them...wierd.
Also in case you don't know about it...fedex has a new ground service called home delivery...if you and your recepient are in 'home delivery' cities this is really cheap. I shipped a set of rims for $30 and a couple scanjet 4c's for $11 a pop, got there in 2 days.