[Suhiir]

The US Air Force has been trying to get rid of the A-10 for years. As an institution they REALLY don't like CAS, that's not to say the actual CAS squadrons and pilots don't see the need/value.
Ever since its' official founding in 1948 the USAF top brass has been convinced the only thing they need is fighters and bombers, "fast and high". And with the development of smart munitions the bomber can finally hit a target smaller then an airfield/small town.

Ground support missions have two major requirements.

1 - Fast response - The tactical situation is extremely fluid, targets come and go rapidly. Using high speed aircraft that require full out airbases to take off/land cannot be as responsive as an A-10 (long loiter times) or AV-8/F-35B (forward basing).

2 - Target Identification - From 15,000 ft you might be able to spot a tank, but who's? Again this is where the A-10 (flying low and slow) can identify it's target and make sure (most of the time) its shooting at the bad guys. USMC and USN aircraft performing CAS missions typically fly at around 500 ft (and frequently lower). They also typically make two passes over the target, the first to identify it and insure their approach/exit doesn't pass over friendly units (in case they hit short/long) and the second pass to actually fire/drop their weapons. The USAF hates to make multiple passes because the first pass alerts the opposition and the second allows them to fire AA weapons. But the USAF very rarely flies below 5000 feet so can't take advantage of terrain to limit opposition AA.

Low, slow, long loiter time, and a large ordnance load is optimal for CAS. But this inevitably means your aircraft ARE going to take damage. The A-10 is a flying tank, and the better CAS aircraft of any type can handle a certain amount of damage and stay airworthy. High speed, high altitude aircraft just cannot be as rugged (usually) because it needs low weight and streamlining.