My question is this: in layman's terms, why is it that Einstein's famous equation showing that matter is just concentrated energy (E=mc2) and matter-energy can be converted into eachother, why is it that it so happens to be the speed-of-light squared (times mass) that is equal to energy??? What is so special about the speed of light, which is just how fast light photons move?

Why isn't it 122,000 miles per second squared x mass = energy or 1,000,0000 miles per second squared x mass = energy???

What, exactly, is so darn special about the speedlimit of photon particles that somehow fits into an perfect equation relating to energy/mass equivalence? Is this just a miraculous, unbeliavable coincidence that the number just so happens to be the speed of light that is squared?

Thanks! I look forward to reading your responses!