During the effort to desegregate schools in little rock, why did eisenhower place the arkansas national guard under army command?

The reason why eisenhower place the arkansas national guard under army command: so that the guard would follow its commanders and prevent violence
If the national guard and the army are under different Jurisdiction, the National Guards have no obligation whatsoever to follow the instruction from the Military officials. By placing them under army command, now they are bound to that obligation.