Mainly because HP doesn't supply 7 Drivers for this Printer. You may try the Vista Driver/s but they may not work with 7. Ideally this Printer is not compatible with 7 and as such should be replaced with a new one.
Plug in just the USB to Centronics Adapter and allow Windows to install any required Drivers. If 7 doesn't recognize this device only without anything attached the USB Adapter isn't Windows 7 Compatible. You can try a different Make or Model of this type of Adapter but try to find one that has a Chip Set Compatible with Windows 7.
You also need to make sure that you get a USB to Centronics Adapter not a USB to 25 Pin Comm Port Adapter as the Comm Port is not a Printer Port and will not work.
You can get Vista Drivers for this Printer from here but be warned a lot of 64 Bit Vista Drivers do not work with 7. I have not tested the 32 Bit version of 7 as it's a dead end technology so I don't know what works with what. ;)
http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?product=58747&lc=en&cc=au&dlc=en&lang=en&cc=au&submit=Go%20?‚?
Col