Tactical and technical advantages on the part of the British combined with tactical mistakes on the part of the Zulus

Prepared soldiers with breechloading rifles in a fortified position with a clear field of fire are difficult to dislodge if you don't have access to significantly superior fire-power. While the zulus did have some guns their force was largely reliant on spears (Assegai).

While accuracy of a general line troop of the period wasn't up to modern standards a reasonable rate of fire still rendered them fairly effective. If you could run 300 meters in a current world record time you could expect to have 6 shots fired in your general direction from each defender. If you aren't that fast or the weight of weapons and a shield slowed you down you could expect more shots. The British fired around 19,000 rounds during the battle

Already you can see that the attacking force faces a problem. The fortified position just adds to them since even once the attacker is close enough to use their spear they are still at a disadvantage against the defending bayonet user.

So that's the advantages that the British had and the problems the zulu force faced.

The Zulu force had also marched about 20 miles before the attack.

The deciding factor though could be argued to have been a tactical error made on the part of the Zulu forces. Rather than a single concentrated attack the Zulu forces carried out a series of smaller attacks that were never large enough to entirely overwhelm the defenders.