pronouns - Any reason Japan's Diet is not used without a definite article (vs Congress, Parliament, etc)?

Obviously, there are instances, but compare for example

"member of Congress" vs. "member of Diet"

The latter, according to Google Ngrams, is basically not used. Likewise, "session of Diet" is not used.

I have read the answers on articles with government agencies, but I wonder if there is some reason that "Diet" in particular resists being used without an article.