Terrain. Simply put, the terrain that ANZAC forces were required to advance over after being landed in the wrong place was not suited to offensive operations. The gullies, ravines, cliffs and the occasional piece of flat ground served only to funnel attacking forces into choke points defended by Turkish forces. There was no room to manoeuvre and unit cohesion was quickly lost as advancing forces had to split to manoeuvre through the terrain.

Now, that said, there was a remarkable level of ineptitude at all levels during the Gallipoli campaign that contributed to ANZAC casualties. One well known example is the Battle of the Nek which took on 7 August 1915. The battle, which was a battle in name only, was to be made by the dismounted Australian Light Horse and was supposed to coincide with an attack by the New Zealanders on Chunuk Bair. The intention was to trap the Turkish forces between to two attacks, a Hammer and Anvil as the Wikipedia page puts it. Now, prior to the attack taking place, Turkish positions at German Officer's Trench (the actual name given to the position) were supposed to be attacked. This position held Turkish machine guns that overlooked the ground to be covered by the Light Horse. Those positions were never taken, leaving the machine guns intact. Added to this, the New Zealanders failed to reach their starting point on time, removing the anvil from the Hammer and Anvil analogy. Regardless, the Light Horse were ordered to proceed with their attack. You know the old adage that things couldn't get worse? Well they did. The naval bombardment that was supposed to preceded the attack was mistimed and ended a full seven minutes before the Light Horse began their attack, giving the Turkish defenders ample opportunity to re-man their defences. When the Light Horse launched their attack, 150 men in the first wave, they were shot down within thirty seconds. Unbelievably, a second attack was launched two minutes later with those men meeting the same fate. A third and fourth attack were also launched, though after hopping the bags, many of the men from these waves quickly took what cover they could find.

The attack at the Nek was perhaps the worst example of military ineptitude from the Gallipoli campaign and post-war analyses of the campaign often state that the most thoroughly planned and executed operation from the campaign was the evacuation, testifying to the poor quality of just about every other operation that took place on the peninsula.

Sources: Official History of Australia During the War of 1914-1918 by Dr Charles Bean Gallipoli by les Carlyon Desert Boys by Peter Rees