I think the answer to this question lies in the focus of this awesome poem and the realistic way in which it is written. Let us remember that a key and central event that fed into what we know as Modernism was the Great War, or World War I, which, through the way in which old values of national honour and glory had resulted in a war that resulted in massive loss of life, created a sense of disillusionment and pessimism and a new realism. Thus we can see this poem as capturing some of the key elements of this movement through the way it acts as a reaction to romantic notions of war and honour and patriotism through its presentation of the grim realism of what soldiers actually endured.
If you look at the very first line of the poem, Owen does much to strip away any idea or impression of war's grandeur. We have a picture in our mind of soldiers as being all dressed in uniform and being strong, young men, proud to fight for their country. The first line presents the soldiers as being "like old beggars under sacks" and "coughing like hags." They are so exhausted that they are "Drunk with fatigue." When death comes, it is not at the hands of an opponent that the soldier has met in battle, it is impersonal, distant, and indiscriminate. In addition, the manner of death is horrific as the gas is shown to torture him. Note the description of the body that we are given:
And watch the white eyes writing in his face,
His hanging face, like a devil's sick of sin;
If you could hear, at every jolt, the blood
Come gargling from the froth-corrupted lungs...
Nothing is spared in the hideous description that we are given to communicate the way in which such notions of heroism and patriotism are actually lies. Thus the poem remains an important example of Modernist poetry through its realistic and pessimistic presentation of the realities of war.