What is important to consider is that E = mc² is the culmination of decades of painstaking scientific research by many brilliant scientists. In the end it lead to the theories of Special and General Relativity: arguably the most profound development in our understanding of the universe of all time.
E = mc² is one of the conclusions Einstein was able to draw, after years of hard work and number crunching for conclusive, airtight proof of all of their findings. It is, in fact, an extremely condensed equation and a mathematical explanation of why this equation is true and the mass-energy equivalence is indeed what it is, would require many pages to explain.
Since us mere mortals couldn’t begin to understand all the theories and mathematics behind it all, E = mc² became General Relativity’s catchphrase as it were. Great marketing, really.