In essence, the central powers could never win a protracted war against the allies, they did not have the industrial output or finances to manage it, they also lacked the access to raw material that would have been needed for a longer war. Throughout the course of the war, the allies spent an estimated $58bn compared to the central powers spending of $25bn (both figures in 1913 dollar value, - Menderhausen, The Economics of War (1941) p 305).

The central powers, Imperial Germany in particular, were heavily dependent on imports of raw materials, this was well known and the plan to blockade Germany had been in place since at least 1905 in case of a war. By 1915 the German imports and exports had fallen to 50% of pre-war levels and the agricultural output was falling rapidly due to shortages of fertilizers. By 1917 the average german was living on a diet of ~1000 calories (Vincent, The politics of hunger : the allied blockade of Germany, 1915–1919). Malnutrition and related issues were becoming severe at this point.

It is something of an over-simplification perhaps but in general terms, large wars end up being a pissing contest in national economy, whoever can piss away the most usually wins (other factors do play in of course, distance from the home theater etc) but it is a good rule of thumb.

The central powers simply ran out of steam. They had less industrial capacity, less manpower and less agricultural production. Their chance was in a quick victory or substancial land gains and a beneficial peace, not in a long war.