I did a study on earwax as part of my degree in neuroscience (Bsc.Neurosci. Nottingham 1997) , and know a fair bit about Sensory Reception in both humans and cats. . .
 The principal constituent components of earwax are complex sebaceous fluids and sloughed off dead skin cells.  The sebaceous fluids are composed mostly of triglyceride fatty acids, waxes and cholesterol esters.
A cat is not a spohisticated creature when it comes to its sense of taste.  Where a human has close to 10,000 tastebuds, a cat has less than 500.  However, they make up for this by having a higher concentration of olfactory receptors. (Estimates vary between breeds, but they have approximately 50 times the number of our 5 million or so odour sensitive cells.)  Like humans though, they only respond to the four basic taste types of sour, bitter, salt and sweet - their response to sweet being the weakest.  Most of our experience of flavour is actually due to smell - it is our sense of smell which determines why different food 'tastes' different.  It is reasonable to assume that cats use a similar blend of smell and taste in determining the palatability of food.
Consider the following.  Cats rarely reject a proffered piece of food on the basis of it's taste.  They use smell as the discriminator to decide if something is likely to be good tucker or not.  Using their sense of smell, the cats subconsciously realise that earwax has a nutritional value - specific olfactory receptors will be triggered by the scent of the fatty acids, the cholesterol, and also the dead skin cells (mostly protein).  Cats are specifically attuned to the scent of animal proteins, hence their preference for meat - also the reason why cats hungrily devour earwax (and also love freshly picked scabs).  The ratio of smell receptors to taste receptors is virtually reversed in cats and humans - as such if something smells good but tastes like shit then we won't eat it, but if a cat likes the smell of it, it has to taste fucking awful before it won't eat it.  A cat will eat small lumps of plasticine if they're covered in gravy.  Try it - it's non-toxic and you can pepper your cat turds with interesting multicoloured lumps. 
The upshot is that humans, with their increased sensitivity to taste, generally reject eating earwax because of its sour, bitter taste (oh come on, everyone has a dim childhood memory of what earxax tastes like, don't they?).  For cats, however, the sour and bitter flavour component of the earwax is barely noticeable in comparison to its wholesome nutritious odour.     QED. 
Disclaimer: The author does not endorse the feeding of plasticine to cats on a regular basis.  Only when they've shit on your bed, coughed up a furball on your brand new novation nova II, or scratched the fuck out of your 200 quid carhartt coat. In fact fuck it, now I think about it, feed the little cunts whatever you want.
b3ta i.d. muffrat