"why can't it be a private venture that only the members get to participate in"

For the same reason that an awful lot of things like Cancer research, or space flight, are publicly funded. There's no good reason for private industry to go there. You wouldn't have GPS without public funds. 

Private ventures don't do things that the don't see lot's of profit in. Why would a company making adhesives, seek out a soybean based adhesive if they're already making plenty of money with what they're doing? They won't. Why would they? Oil companies weren't seeking out Ethanol or Biodiesel. We did the basic testing on biodiesel long before any engine maker or fuel company was interested in it. That simply wouldn't exist without the soybean checkoff. Like it or not, it takes political clout and some funding to get alternative things done. I swear, some of you guys want to go back to the 1930's. 

 Right now there are checkoff funded researchers working on dye substitutes based on Corn. But you don't know anything about that, why would you? And why would a private venture be doing it? They wouldn't? They're going to fight to the end to do what they're already doing.