I'd imagine the driving force is still cost.
HUDs are an optional extra on the type I fly, for the Captain only, primarily used on the approach to reduce the limits on visibility and DA.
If the units have a limited life of operation due to bulbs/burn in or similar it makes sense that they would be used only in the descent and approach phase of the flight. As such their use as a PFD wouldn't make sense as the pilot would have to learn 2 different scans. One using head down instruments and one using a combination of the HUD as a PFD and the other displays in an unusual display mode.
Even if the HUD were used as a PFD would a HUD failure ground the aircraft? The thinking being that you've lost a primary system and are relying on a backup (even if you are back to using what everyone else is) Not sure if you could class it as a deferable defect
The units available on the type I operate are steam driven monochromo and appear to have a relatively low resolution, so I wouldn't see them being very useful in displaying all of the information from the PFDs in a clear manner.
As the technology improves and filters down from military applications I'm sure the reliability will increase and the cost fall at which point we'll see HUDs becoming more common place on both sides of the cockpit and for a larger range of uses.