I like to think of it along these lines:
Metal and wood weapons can be tempered through magical, physical and alchemical processes to be resistant against fire, cold, acid and electricity, along with the sheaths that hold them.
But when it comes to making the weapon vibrate to a degree strong enough to damage another being, it will slowly(or quickly*shrugs*) and cumulatively damage the structural integrity of the item. And these types of items can't simply regenerate that damage, nor can it be polished or resharpened into good working order. It would have to be reforged entirely if metal, or s...ped if wood.
As I believe the weapon enhancements never stop working, even when not in use, this is a continuing effect.
It might never show cracks or start to warp, but given time and usage in battle, it would simply shatter or break one day.
This would make it a very powerful, but very shortlived weapon.
As for Positive, Negative, Divine, Unholy damage, those are the most pure forms of energy in existance. All else it pretty much junk next to these.
How would anyone not in the epic range, or aided by a deity, be able understand and harness such raw powers without destroying yourself in the attempt?
Edit : It's true that mages and clerics can use this energy at low lvls, but you are only skimming a little bit of power to use for spells, but to channel a continual effect into a weapon would be a bit much for anyone to accomplish.
