Several interconnected reasons:
Fuel shortages. Self explanatory. No fuel = less combat ability, plus the huge factor of less fuel to train replacement pilots.
Poor pilot rotation. Combined with #1's effect on training effectiveness, this means that losses in pilots could not easily be replaced.
Slow development of new aircraft after 1943. Allied bombing, mistakes/stubbornness in policy and specifications, meant that after the Allies caught up to the introduction of the Fw-190, Germany didn't have a truly superior fighter aircraft in large numbers. This, together with the Soviet Air Force catching up on the eastern front in very large numbers, and costly fighting against Allied bomber raids, led to an unwinnable battle of attrition.
Masters of the Air by Donald Miller is a good source on the changes in air combat on the Western Front, from the Luftwaffe being an effective and formidable defense in 1942 to their collapse by mid/late 1944.