Asking why only priests can consecrate the Eucharist is a little like asking why only congressmen can vote on bills or why only brain surgeons are allowed to do brain surgery. It is what their function is. The real question is why should there be a ministerial priesthood in addition to the universal priesthood all Christians share. To understand this, let's back up for a second.
Human society requires a division of labor.
Within civilization there is no such things a true jack of all trades. The mountain men of the 19th century may have been rugged, but they also had amazingly short lifespans, no more than a few years once they left civilization. Man is a social being. There is a good reason for this. The holy things are the most important things there are, meaning that they of all things require a specialist to minister them.