I think it's because Ford may well have produced the most vehicles of any single brand in the world. Although it's possible Toyota or some other big car brand may hold the record, but Ford is pretty much the biggest name in both the American market and the European market, and the majority of films/TV shows on this site are American and European. Ford has become the leader in several market segments (the Crown Victoria in the US police car and taxi market, the Transit van in the European van market) and has produced various big sellers over the years like the Mustang, Focus, Mondeo, Fiesta, Taurus, Explorer and the Model T which started it all.

So since there have been so many Fords around during the last hundred years, naturally more Fords appear in films then other car brands.