There were a number of factors that led to this.  Among those factors were:

• Lack of a democratic tradition.  Japan had never really been democratic at any point in its history.  Therefore, it was hard for it to remain as a democracy.
• The desire for empire.  Japan saw itself as a powerful country that deserved to be an imperial power.  This militaristic vision helped to cause people to feel that the military should have more power.
• A faulty constitution.  The constitution of Japan gave the army essentially complete power over the governments if it chose to use it.  Any army minister could bring down a government by resigning his post.  The Army could then prevent the formation of any new government by refusing to appoint a new minister.  In this way, the Army could force the government to do what it wanted and to appoint the sorts of officials it wanted.

All of these factors led the military to become dominant in Japan in the early 1900s.