At top level the Sports Cars category, unlike F1, was always heavily based in manufacturers.
In F1 you have basically one resident manufacturer (Ferrari) but that happens because Ferrari was the
Scuderia Ferrari before being the
Manufacturer Ferrari (Ferrari Auto S.p.a. I think). In the eraly years you had Mercedes but they weren't a Scuderia first. And you had many others every now and then, but none like Ferrari (with the possible exception of Colin Chapman's Lotus Team and Lotus Cars when they both belonged to him).
In Sports Cars, however, you ALWAYS have and had manufacturers driving the discipline forward. Much more than Formula 1 - and in this I don't agree with many other posters - it is at Le Mans and at similar events that the manufacturers really "flex" their muscles. So, when you don't have manufacturers in Sports Cars the discipline faces oblivion and therefore lack of sponsors for the privateers.
And this is bad because - and in this I totally agree with Ardius - it's in the presence of privateers that also resides the beauty of sports cars as a racing discipline. Not privateers like the "non-manufacturers" of Formula 1. REAL privateers. Le Mans and both Le Mans Series is where, at top level, you can still find "gentleman-drivers". Actors, musicians, businessmen, occasional racers, whatever. And low-budget teams that do show up to race, some very professionally, some not (never forget the awesomeness of the Japanese Lamborghini Owners Club. They do try, year after year
)
But Sports Cars need the manufacturers. Be it sports cars makers like Ferrari and Porsche, be it big companies like Audi and GM and BMW and Toyota and Peugeot and Nisssan ... they're fully part of the game and therefore they're missed when absent.