Meh. Weapons and armor as well as the the soldiers wielding them and the empires employing have always been in somewhat of an arms race, as such the most effective weapon and best armor completely depend completely on the state of technology and economy at each time frame.

As such the reasons why Orcs would use blunt weapons would be heavily influenced by their culture and not so much the supposed superiority against any sort of armor.
While it is somewhat of a cliche, to say that Orcs use heavy weapons because they are barbarians probebly isn't that far off. A culture without a strongly organized hierarchy will create a culture where personal prowes and individual abilities are favored over close cooperation and strict formations will favor weapons that utilizes this space and the stamina of a warrior.

I wish I had some examples to pull out of wikipedia, but I don't.

edit:

Having superior protection, and thus having no need for close formations can also allow for lose formations and weapons that use this space. At the same time this also means that heavy weapons are preferred to penetrate this armor.

As such the Barbarian cliche can be turned around, the Orcs use heavy weapons not because they're unorganized but because their advanced armor allows for it and requires it.

I've seen them using claymores in Oblivion sometimes, but for the most part, they do use only hammers and axes.

Hammers and axes have other uses, other than killing which is the sword's weakness. Since the orcs lived hard and rough lives, the demand for multipurpose tools is higher than the specialized sword, which is used only in war and battle. I can only see the warlords and the higher ranked orcs wielding swords a lot more often than a normal orc, more so for status.

To use a sword most effectively would also seem to demand more time in practice and in training, things that poor peoples could scarcely afford. A sword also likely requires more fine maintenance. A sword without it's edge is useless. A hammer has no edge to keep sharp. You can still bludgeon someone with the heavy ax, even if dull. (Though I should note that a dull ax is somewhat less safe for the wielder, at least if you are chopping wood. Still, I reckon that with an ax there is also less edge to file to sharpness, compared to a sword.) To summarize, the sword is a weapon for the aristocracy and landed nobility (the oppressors, if you will), while the hammer and ax are egalitarian weapons. And I suppose it doesn't hurt that Orcs in TES are strong and stocky, well able to deliver crushing, disabling blows. A sword (spec. a one-handed sword) might be better suited for quicker, more agile folks who need to deliver more precise blows.

Sonce their berzerking ablity gives a large strength boost at the expense of agility, it makes sense that their favore weapons rely on strength rather than the finesse of swordplay. Though an axe can be handled with finesse, but it don't requires it.

And we can probaly thanks decades of axe-wielding fantasy barbarians for it too. Even if the brarbarian protoype (Conan) used swords most of time.

The mighty Lord Rugdumph had a sword.