The other possibility I can think of is one that is a little sad (and hopefully not true)--running makes money, track and field does not.
If Nike went away from track and field and others didn't step in to fill the void, it would mean that these companies have realized that the real money is in running (jogging, road races, healthy living, etc.), not in track and field.
Sometimes I wonder if the only way for a company to really make money on track and field is to dominate the market as Nike has done, to have your spikes on the feet of half of the competitors in every high school meet. Running is a different story. Lots of people have running shoes. But short shorts, spikes, etc? Maybe the only way to really bank on these items is to get half of the market share because there just aren't that many people buying spikes (compared to how many buy the Gel Kayano and Brooks Ghost).
What gives me hope, I will say, is how heavily New Balance and others are investing in lightweight, elite spikes, racing flats, and meet sponsorship. Hopefully companies have decided that track and field is an area in which they can make money. Considering this last point, I actually am really happy with where track and field is right now and its potential for growth.