What made U2 so popular?

I mean was it really just the sensation of a catchy tune? Was it because "The Edge" played a guitar that was drowned in effects and delays? I mean seriously! What did these four guys do to sell out every football stadium and have record breaking ticket sales for every concert? Is it Bono's political and moral activism? I just want to understand! I know their music is catchy, but there has got to be something else that has caused them to become the most popular (and in my opinion, slightly overrated) to every continent? Now don't get me wrong, I have NOTHING against U2 as a band or artists, but I just want to understand why more bands who appear to have much more talent do not get the exposure that U2 has gotten all these years. Just please enlighten me. PLEASE!!!