To prepare for use, the barrel holder was required to be removed from the slab-plate and then all 306 empty shells were removed. Then it was proposed to install new cartridges in the barrels. Having reloaded the trunks of one module, the technicians had to do the same procedure with three others.

You have to disassemble, unload, service, reload and then reassemble each barrel. Working on this thing would’ve been awful and rearming the system would’ve taken forever. On top of that, it was basically useless:

Thus, shooting from the XM215 MBG installation led to very specific results. Light bullets with insufficient speed immediately after leaving the trunks fell under the air flow from the rotor. This combination of factors led to the fact that the bullets flew in a downward trajectory and fell to the ground closer than expected by the calculations. In addition, depending on the height of the helicopter, by the time of the fall they could lose most of the already small energy. Because of this, the effective aimed fire range was limited to only a few meters, after which the behavior of the bullets became unpredictable.

And a hazard to your own troops:

Tests have also shown that the XM215 Multiple Barrel Gun system can pose a danger to its own soldiers on the ground. In certain cases, when shooting from a low altitude, uncontrollably falling bullets could hit not only the enemy, but also their fighters. This problem was also aggravated by the characteristics of piloting helicopters at low altitudes: even small movements of the helicopter could lead to new risks.

reference