Because...

To get modern-day Zulus to play the Zulus in the battle the producers had to get the cooperation of the then-ruling King of the Zulus. Seems the King didn't dig the true story as related by the nitpicker, that the Zulus assaulted a vastly outnumbered force and then bugged out when it turned into a fair fight. To get the extras they had to rewrite history to put in the "saluting fellow braves" scene. A lot of the film varies from the facts for dramatic reasons. There were no lulls in the real battle but they're in the movie because they were needed to allow the actors to have dialogue to develop the plot. Similarly, Commisary Dalton was no wimp and Private Hook was no malingerer, but the movie charcters worked better in the script than the real people. Finally, no one said they were all welsh. As one private said "This is a Welsh regiment, man. Although there are some foreigners from England in it, mind."

 

Not a Welsh regiment

The regiment did not become a 'Welsh' regiment until many years later. Although it was based in Brecon in South Wales and called the 24th. Regiment of Foot and later became the South Wales Borderers, before that it was the Warwickshire Regiment. And they wouldn't have sung 'Men of Harlech' if that ever happened, which it didn't. They would more likely have sung the regimental march. Most of the men serving had never even been to Brecon. This is not a nitpick of the nitpick just a comfirmation and bit more info.

 

They also left out the dog

Unlike Documentaries, Historical Dramas often add, delete, or alter scenes for dramatic effect.

For example, the bit where Lt Chard supersedes Lt Bromhead is wholly invented. When the actual commander at Rorke's Drift went off to get help, he simply left Chard in charge. No confrontation. End of story. But also no drama.

Still, taken as a whole, Zulu does provide a reasonable good account of Rorke's Drift. Those interested in a more accurate account of the events might click on this link to a history of the Anglo-Zulu War .