That's a good question, and I'm not entirely sure why. I read it was hemp rope from the Royal Navy, so it may have been a bit of an ad-hoc/improvised armor for lack of better options.

https://norml.org.za/hairy-marys-hemp-armor/

[–] tburkhol 10 points 1 year ago

2nd Boer War was 1899, so there wouldn't be any synthetic fibers. Steamships were in wide use, but there were still a decent number of sailing ships, especially for local/coastal trade. Essentially all marine cordage at the time would have been hemp or Manila (banana), alternatives being cotton or linen, which aren't very durable. Manila rope is often called "Manila hemp," even though it's a different plant, and I can see soldiers not bothering to make clear distinction. True hemp is somewhat stiffer.

Either way - hemp or Manila - would have been plentiful, dense, and pretty effective at slowing projectiles & shrapnel. The image looks like those ropes are probably 2-3" diameter. Heavy stuff. Probably much easier to drape a bunch of 3" rope over your locomotive than to build a 3" timber frame around it. Nevermind the equivalent iron armor.