Because you cat is either English or Australian :)

Or it is bacause of the salt content. Marmite is particularly salty and most animals like salty foods, even to the extent there is a response called Salt Hunger.

Cats like salty things, for example; cheese sauce, marmite (on toast in my cat's case), butter (leading to a largely perforated butter lid) and other things left on the kitchen sideboard.

A more truly odd question is the love that my cats, I don't know if it's the case for everyone, have for earwax, try sticking your finger in your ear, not so it has visible wax, just to get the feeling of it, now offer it to the cat...

Possibly because of the yeast and the salt. It may be that your cat's diet is deficient in Vit B, but in the same way that we enjoy salty crisps, your cat may have a similar fondness.  I wouldn't let it have more than a i/4 teaspoon a day though.

Whilst cats like salt their sense of smell predominantly governs what they like to eat.  The 'meaty' smell of marmite is generally sufficient for cats' curiousity to get the better of them to taste yeast extract products.

The experience with my two cats is that they are both attracted by the smell of yeast extracts but that tasting too much at once turns them away.  Like me they prefer some of the milder tasting yeast extracts.  Get the quantity right and they'll even take their de-worming pills without fuss if the pills are smeered with it first.  No pill popping contraption, no having to hold them down or hold their mouths open, they just eat them like treats (although one trick is to tempt them with a little to lick off your finger first).