Question about confidence intervals (intro stats)

[kingofmirrors]

Say there's a project that will only get approved if 80% of the people polled support it. You gather some sample data and create a confidence interval that comes out to (50%, 90%). Why can't you then say that based on the confidence interval there is a 25% {(.9-.8)/(.9-.5)} chance the project will be approved?

(note: if you can, it was my stats teacher who said you couldn't, but didn't bother to explain why not).