Money. OnTrac is substantially cheaper than UPS and FedEx in their respective service areas. This was always a sore point amongst many people when I worked at Amazon - the company internally prides itself on being absurdly pro-consumer, and the use of OnTrac (among other sketchy small-time delivery operations) was a very big slap in the face on that philosophy.

It's a pretty blatant case of acting against your customer's interests to save a buck, something Amazon has traditionally prided itself on not doing.