Near the beginning of his essay, Miller posits:

"... if the exaltation of tragic action were truly a property of the high-bred character alone, it is inconceivable that the mass of mankind should cherish tragedy above all other forms..."

This is a strong point about why people like to read tragedies and view tragic plays. What Miller means is that high-bred characters, royalty, nobles, the rich, and others of "esteemed" social standing do not have a monopoly on experiencing the catharsis provided by the unfolding of a tragedy. Into each life some rain must fall, regardless of whether one is a king or a "commoner." The common man is a suitable subject for a tragedy because he will inevitably experience one—his journey will (arguably) be more resonant with an audience than that of a figure unlike oneself.

As he ends the essay, Miller concludes: 

"In them [tragedies], and in them alone, lies the belief-optimistic, if you will, in the perfectibility of man."

He draws the distinction that pathos is for pessimists and that, in the struggle of a tragedy, the common man has the opportunity to claim "his sense of personal dignity"—to find meaning in whatever tragedy befalls him. One needs to only consider John Proctor, the protagonist of The Crucible to understand what Miller was getting at in this argument. Though John Proctor, a lowborn farmer and occasional sinner, pays for his self-actualization with his life, he finds satisfaction in clinging to his faith, honesty, and integrity in his ultimate stand against corruption. Moreover, because The Crucible has been staged around the world in different political and social contexts than the one in which Miller wrote it, his argument that John Proctor's situation is a universal and timeless subject proves true. 

In his essay, "Tragedy and the Common Man," Arthur Miller makes one very strong point regarding the common man's suitability for tragedies.

According to Miller, the common man is best suited for the tragedy based upon the following:

The quality in such plays that does shake us, however, derives from the underlying fear of being displaced, the disaster inherent in being torn away from our chosen image of what and who we are in this world. Among us today this fear is strong, and perhaps stronger, than it ever was. In fact, it is the common man who knows this fear best.

What this means is that the common man is the one who truly understands fear and, therefore, is best suited for the tragedy based upon his understanding of fear. Man, forever in search of the morality (based upon their own perceptions), is most capable of finding himself for himself. It is up to no other person but the character in question to come to the conclusion about his own morality and the extent he will go to in order to find it.

Men who are not "common" fail to have the same mindset as common men. They are more impressed with the abstract qualities based upon the fact that they define life for themselves only--in a way they are above traditional thought and fear. Common man, on the other hand, accepts the "condition of life" and both flourishes and self-actualizes.

Therefore, the best way to define the common man's suitability for the tragic role is to use Miller's words alone: "the common man knows fear best."