here is what I could find (nothing official though but it sounds plausible)
Nikkor is the standard designation for Nikon's lens lines. The first 'Nikon' lenses were the Series E lenses, which were a lower-end consumer line frome the late 70's and 80's. The only lenses that don't say Nikkor are the consumer grade lenses. Most point and shoot lenses say Nikon. The marketplace has blurred that distinction. The generally accepted keyword phrase is 'Nikon Lens'. With most searches you will search for Nikon Lens and Nikkor won't even show till you get to the individual listing for the lens,
Also found this which might explain it further. Again This is all just what I read that individuals were saying but to me it had the ring of truth
Ikon is a trademark owned by Zeiss, so Nikon were wary of infringing which is why they sell their lenses under the Nikkor brand. Then Ikon disappeared for many years, and now it's back again as a camera body rather than a range of lenses.