Perhaps the fact that Discovery seemed to appear behind Saturn as far as line-of-sight/sensors on Earth were concerned?
Also, perhaps once Discovery did appear, the Earth sensors didn't identify it as something to be concerned about due to it's 23rd century makeup.
They may have thought it was just a very old ship of little concern until Discovery was right on their doorstep.

Also, perhaps once Discovery did appear, the Earth sensors didn't identify it as something to be concerned about due to it's 23rd century makeup.
They may have thought it was just a very old ship of little concern until Discovery was right on their doorstep.
