Well, for beginners, Bosnia was a member state of Yugoslavia before its collapse. Yugoslavia didn't have regional armies as such, it had the JNA (the Yugoslav national army) - which, once the war began, was entirely in Serbia's hands as the military headquarters were in Belgrade. The ARBiH didn't really exist until Bosnia itself was invaded, and was put together in quite a spontaneous manner by regular civilians (hardly any members were fully trained soldiers). So one big reason is therefore that the BiH army didn't actually exist until the beginning of the Bosnian War, as the bulk of the JNA went to the invading Serbian force.

Second of all, as well as being put together in haphazard fashion, the ARBiH had an arms embargo imposed upon it by the United States and other western nations, meaning it couldn't be directly given any arms. Most of the weapons the Bosnian army had were illegal ones which were smuggled into the country via aircraft or through the Dalmatian ports (Ploce and Split) - these consisted of light arms and nothing huge. The army also had virtually no heavy vehicles, and almost all the tanks and APCs that they did have were captured from the Yugoslav army during firefights.