I have thought the same thing in the past so I have googled it a bit. The Wikipedia article on the subject gives quite a good answer.

You have to go back to the World War One. The British wanted a design that was strong and that did not use a lot of material. Ultimately, the helmet was designed to protect the wearer from shrapnel from shell-burst from above, which was the most likely direction it would come from when in the trenches.

On to World War Two. Simply, the helmet was kept, although with some comfort modifications.

A new helmet replaced it in 1944, and as you can see from the photo here it is closer to the designs of the US and German helmets.