How are the other branches doing with officer retention? If the Marines, Navy and Air Force aren't having the same problems then it could be a cultural problem with the branch, not an incentive issue.
Part of the problem with the Army is that they want to grow the overall force -- while the AF and Navy have been shrinking recently. Not only does the Army want to grow by 30,000 troops, but the design of the new brigades have relatively more jobs for lieutenants and captains.
This article doesn't talk about the Army reserve components -- I understand they have a worse officer shortage than the active component.
Part of the problem is branch cultural: the Army has always been the low dog on the totem pole. The Air Force has the incentive of letting the people they really care about fly jets, the Marines have their elite esprit d'corps, and the Navy has always been the most socially elite branch (due to the connection with the upper class on the East Coast dating from the days of sail - sailing remains a connection). The Army has always been the guys who came in huge numbers of average quality who get the job done. Not much glamour. Not as many neat toys to play with. And, since the cavalry was dismounted, no logical connection with the upper class (the country's best polo players used to play for various Army teams, or played against them and met as social equals.)