Short answer vapor production, the 510 was a huge innovation a few years ago, remember this was before the ego, before cartomizers even you pretty much had the choice of attys with carts or dripping on your attys hell drip tips might not have even been on the scene and long before VV mods the only mods available were big bulky box mods or something you built yourself. Simply put in terms of flavor and vapor production it out performed damn near anything out there, at the time the only thing that came close was the 801 (pen style) which was much larger and odd looking unless you had a janty stick (if anyone remembers those and those were odd looking but much more compact).
tl;dr: When the 510 hit the market it outperformed the competition in a much more compact ascetically pleasing form factor. It was regarded as the best out there and widely adopted as the new universal threading so to speak from that point on.