Because otherwise, and you kill Rolf, you'd have a very angry, murderous Galmar Stonefist coming at you with full, no holds barred, fury of the Nords. Galmar isn't exactly one to go, "I will fight anyone who threatens my family! Oh, you...you killed my brother? Ah well, no matter, want to help fight for Ulfric?"
It was to prevent any breaking of the Civil War questline by offing Galmar early, or have people wonder why Galmar doesn't seem to take his brother's death a bit more personally given his personality.