Because 5V is the only voltage that is universally available on eSATAp. To quote Wikipedia:Yeah, but the adaptor doesnt have a buck convertor to provide even the 3.3V let alone the 12V.
There are only two versions of this port. Most laptop computers do not have 12 V power available, and have an eSATAp port which provides only 5 V. Desktop computers, with 12 V available, have a port with two additional pads, placed against the plug's "horns", which provide 12 V. Some manufacturers refer to these ports as eSATApd, where d stands for "dual voltage". Some devices, such as 2.5-inch drives, can operate off the 5 V supplied by laptop eSATAp ports. Others, such as 3.5-inch drives, also require 12 V; they can be powered from a desktop eSATAp port, but require an external 12 V power supply if used with a laptop computer. This can lead to confusion if users are not aware of the distinction.