The A-10 cannon is not comparable with that of a modern tank. Just compare the numbers:

A-10: 30mm Gatling

M1 Abrams (and other comparable tanks): 120mm (Russland's newest tank has a 140mm gun as much I remember)

That is a huge difference! The only advantage of the Gatling is the high firing rate -- but it is definitively no match for the armor of a modern main battle tank. I also never implied that. When the A-10 was designed, tank armor was less advanced and today the A-10 is mostly fighting against light to medium armored targets. Many countries even in their regular army have still very old tanks in service.