text: You know what you are asking here can't reallly be answered. The reason ATP is the main energy source is because that's what happened through the process of evolution. And, as you know, there is no way to determine WHY something happened when it comes to evolution -- despite what the intelligent design advocates will tell you. All you can say, based on the ubiquitious nature of ATP's usage as the main energy carrier throughout most living organisms, is that ATP became the preferred energy carrier very early on in evolutionary time. Probably going back as far as the last universal common ancestor (LUCA).