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 & tart, a lot of their competitors went for a similar flavor profile.

There are 3 main reasons that mainstream energy drinks have a bitter 'bite' to them. #1: Taurine is a most important additive that helps to potentiate the caffeine to reach the bloodstream faster, as well as speed up the metabolism for most people. However, Taurine is acidic, which in turn means it is also bitter/sour. Combined with citric acid, sodium citrate, vitamin c, tartrate, sorbic acid, benzoic acid, and the b-vitamins as well. Not all have the same exact ingredients, but they all contain a lot of acidic-bitter flavors. #2: The abnormally sour 'bite' actually stimulates the brain to release adrenaline and/or serotonin in most people, in the same way spicy foods do, generally. It's a cheap 'extra kick'. #3: Many people have a very mild 'addiction' to sour/sweet flavors. Like the candy, Sour Patch Kids, which is the most popular non-chocolate candy on the market, people have a strange enjoyment of the sour-sweet combination. Much like salty and sweet as well. They compliment each other. However, drink in moderation because they are very unhealthy unless you buy organic versions.