Krav Maga's emphasis on real-world self-defense and potentially lethal techniques makes it less suitable for the regulated environment of MMA competition. 1. Krav maga techniques are designed for survival and often escape rather than "victory" or dominance.
Because well taught Krav Maga is so comprehensive in what it teaches, it makes it very effective in a street fight situation, where anything can happen (it's also why you really need an instructor with a huge depth of experience and knowledge you don't learn to defend effectively against all of those scenarios ...