Because booting from such drives is not supported. You can use an external
SATA enclosure with a SATA drive in it if you use an external SATA or eSATA
port to connect it to your motherboard. There are also ways to use scsi
drives externally as boot devices.
It has something to do with how drives must be connected to the PCI bus in
order to be detected in time for booting the system at startup. USB devices
are on a USB bus and not the PCI bus. The BIOS in many systems now have a
setting for usb boot but the usb enclosures do not have firmware to do it
and the usb ports on motherboards do not support it because the USB bus does
not support it. The operative statement from MS is: (from
http://www.microsoft.com/whdc/device/storage/usb-boot.mspx)
"Current versions of Windows should not be installed to USB hard disk drives
because Windows does not support USB hard disk drives as the primary boot
device."
The article then continues with proposals to the standards committee:
"This paper introduces the expected industry participation related to the
following:"
You can read the details from the article linked above.
The international standards committee on this subject is still working
towards the standards needed for everyone to be on the same page so that usb
booting can be implemented. Microsoft has said that they will not have usb
booting in Vista but are looking at it for future releases of Windows.