Why Do We Evolve to Taste Glutamic Acid?

If a naturally occurring food has free glutamate in it, that's because it is full of protein. The reason we eat protein is to get the amino acids that comprise proteins. There are 20 different amino acids out there, and glutamic acid isn't even a particularly important one as far as diet is concerned. Why we evolve to be able to taste glutamic acid, and not one of the other essential amino acids like isoleucine or tryptophan, seems like a weird choice.