I know that "einfach" means easy but I always find it in exclamative sentences such as "Das ist einfach genial!" I really want to know the reason for that.
Einfach is a sort of emphasis / amplification here. It will occur in conversations much more likely than in writing. You could replace it with echt, super or something similar; I have the suspicion, that one of the functions is simply to have more words available to put vocal emphasis on. This is supported by the observation, that a slightly more intense version could be:
Das ist einfach nur genial!
Nur (as einfach) provides no essential addition in this context, but an additional syllable.
It can be compared to the english "simply", or "just".
"Du bist einfach so schnell!" = "You are just so fast!"
http://www.oxforddictionaries.com/definition/english/just (see point 4.1)
"Really; absolutely (used for emphasis): they’re just great"
In this usage, "einfach" is a modal particle. It conveys the speakers' attitude towards the content of their utterance and covers different functions:
a) Intensification, as in the example you quote.
b) Dissatisfaction, as in: "er hat mich einfach ignoriert" ('he einfach ignored me').
c) Awareness that something/someone cannot be changed: "er ist einfach dumm" ('he ist einfach stupid'). This function can sometimes overlap with a) though.