42 simple said
people worked on gnu to get an open-source OS
other people started to work on hurd to give the OS a kernel (basically the hearth of an OS)
then Linux got made and people liked to use that as a kernel for the GNU operating system instead of the still in development hurd.
Linux became "popular" and most people forgot that hurd exists. (some humans still work on it though, and version 0.7 got released this year)