Easy it's a better lens.
Why would anyone buy a 17-55mm is usm efs lens at £500 when they have a 18-55 already. The answer is image quality, your current lens will not be as good as the 17-55 usm by quite a bit.
Hence why people buy the 50mm as it's a very sharp lens, it's lighter than yours, and crucially has many more f stops which means you can take photos in lower light conditions.