Why is there no widely available EEE vaccine to protect humans?

BOSTON – Following the death of a man in New Hampshire from the Eastern Equine Encephalitis or EEE virus, many are wondering what they can to do protect themselves from the mosquito-borne disease. While there is a vaccine for animals, there is no widely available one for humans.
EEE is transmitted by mosquitoes primarily to horses (hence the “equine”) and to humans. Currently, there is a vaccine to prevent infections in horses, but there is no government-approved vaccine to prevent illness in humans.
Scientists at the National Institute of Health did develop a human EEE vaccine that was found to be safe and effective, but it’s been reserved for use in laboratory workers and other people at high risk of exposure, like military personnel, and not commercially available to the general public.
Why? Even though the effects can be devastating and even fatal, the infection is incredibly rare. According to the CDC, there have only been four cases in the United States so far this year. And to bring a vaccine like this to market could take hundreds of millions of dollars to perform the clinical trials required to gain FDA approval, which companies would be reluctant to invest unless they could recoup that investment through widespread sales.
In the meantime, you can significantly reduce your risk of getting bitten by mosquitoes by wearing long sleeves and long pants, using insect repellent containing DEET, and staying indoors during dusk and dawn when mosquitoes are most active.    
The town of Hampstead, New Hampshire, where 41-year-old Steven Perry died, will be closing fields early Friday evening so they can spray for mosquitos.