Why the common basic taste of energy drinks is tutti-frutti?
A lot of the defining ingredients of energy drinks have very strong flavors. Caffeine is quite bitter, B vitamins can be very sour, and Vitamin C is tart. A strong sweet-tart flavor is one of the most palatable ways of masking all of these flavors. 

There's also a certain amount of brand copying going on. Red Bull was probably one of the first big brands on the market. Since their flavor is cloyingly sweet &amp; tart, a lot of their competitors went for a similar flavor profile.