Prismatic shards are on the AH for less than 1g.  There are no blues in Lich King that vendor for less than 1g.  This means that players are taking something for which they can get multiple gold and instead getting less than one.

You might be saying, "But I need shards."  Okay, so then why are they on the AH?  If an enchanter is burning gold for shards for his own enchants, then they won't be on the AH.  Alternatively, if you need shards, buy them on the AH rather than making them.

 Similarly, low-level essences aren't particularly valuable anymore.  Incidentally, this makes them a great way to level enchanting.  Weapons tend to make a lot of essences rather than dusts, so they're also often better to vendor rather than disenchant.

Of course your server might be different.  Maybe you're on an RP server that only uses shards as currency.