It's because of the booming economy

Um, no it isn't...

From the NY Times:

Even after reducing its recruiting target for May, the Army missed it by about 25 percent, Army officials said on Tuesday. The shortfall would have been even bigger had the Army stuck to its original goal for the month...

...But the news could have appeared worse. Early last month, the Army, with no public notice, lowered its long-stated May goal to 6,700 recruits from 8,050. Compared with the original target, the Army achieved only 62.6 percent of its goal for the month.