Why do the men use spades rather than bayonets?

The spade is handier, heavier, and more effective. From the text:

But the bayonet has practically lost its importance. It is usually the fashion now to charge with bombs and spades only. The sharpened spade is a more handy and many-sided weapon; not only can it be used for jabbing a man under the chin, but it is much better for striking with because of its greater weight; and if one hits between the neck and shoulder it easily cleaves as far down as the chest. The bayonet frequently jams on the thrust and then a man has to kick hard on the other fellow's belly to pull it out again; and in the interval he may easily get one himself. And what's more the blade often gets broken off.