It's due to the fact that GoogleBot and other search engine robots were not made to go around decompiling your .SWF files or viewing your website as if it were human. It can only read what HTML code you present it with and index it. For Flash sites, make a fall-back HTML page available, loading up automatically when Flash cannot be used. It'll at least help to get things indexed.
depending on how you created your site googlebot may or may not be able to index it, like others have said in this thread it is due to googlebot only being able to read html.
however if produce the site from inside of adobe flash (I believe its file>export>html) it will fill in the meta tags etc for you so that your site can be easily indexed.