Good question, but unfortunately there is no good answer. Although both glutamate and aspartate have been considered as excitatory neurotransmitters in the CNS for deacdes, the possible neurotransmitter role of aspartate remains controversial to this day. It has been claimed that in contrast to glutamate, aspartate is not transported into synaptic vesicles. So, the release mechanism of aspartate still remains unresolved. Several recent studies, however, indicate that aspartate is released from certain synapses.

 Thanks for your answer Abram. I didnt know that aspartate can also act as a neurotransmitter. But still, what is the explanation for  glutamate being evolutionarily selected as the major neurotransmitter in the nervous system?