Who knows? I don't think it was a marketting strategy, because Nikon lost points with pros by waiting. This means that they lost visibility in front of wannabe amateurs who buy what they see more pros using.
(Sidebar... I just attended a music fesitval and many SLR's were also in attendance. I saw about 20 or more Nikon DSLR's of at least D80 or better level. I only saw 2 Canon's. Only one known pro was using a Canon Mark-something. The rest were using Nikon's.)
I think Nikon had committed to the DX size sensor and felt that they could maximize the results from that format. I mean, making cropped lenses is a reaction to the popularity of the DX sensor size (APS-C). Why would they take 50 years of lens design and start all over if they didn't think DX was the wave of the future when the full-format lenses work just fine on a DX sensor?
After Canon's obvious success with the FF sensors, I think Nikon saw the light and jumped on that bandwagon.
There may have been in-house issues with the design team. This is PURE speculation, but think about it. Suppose you were the design leader and you committed to a cropped sensor saying that it can be "just as good," and then some upstart said that you were wrong. It might take a year or two to change your mind. After seeing the success Canon had with the move to FF, maybe Nikon had no choice.
A better question might be why it took Nikon so long to bring CMOS sensors into their prosumer level. (ie: The D300) They have used CMOS sensors in the top of the line for almost 4 years now, but the D300 is the first one aimed at the advanced amateur.