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

Solution 1:

[Is there] any reason Japan's Diet is not used without a definite article... ?

Well, sure. As the answers here mention, people omit the direct article from locations when they're being conceived more as a state or condition (home, in jail, to church, at school). That reframing happens more when a place is in common reference but some cultural differences among dialects pop up: Americans are at home on their roads and talk more familiarly about college than university; Brits call their colleges unis and are overly familiar with their hospitals.

As some of the commenters above mentioned, a similar effect is apparent in discussing Congress and Parliament. Rather than continually refer to them as periodic common-noun gatherings of people requiring a direct article, they're often reframed in a way that enlivens and streamlines discussion. In this case, they're not really a state or condition; it's more that they're being treated as a proper-noun name. The Congress of Vienna, the United States Congress, (less often) the Congress of the United States, but of Congress, in Congress, &c. At least some of this is an effect of personifying the government.

'Diet' is latinate and unused by the legislatures of the major English-speaking countries, so it's going to trend formal. Even so, the Russian and Japanese diets are sometimes treated similarly, and of Diet and in Diet do appear. Pace @BoldBen, there are plenty of English-language materials on Japan these days and it looks like they simply refer to their 'members of Diet' as 'Diet members' and 'sessions of Diet' as 'Diet sessions'. Meanwhile, we have 'Congressmen' and occasional 'Congresswomen' but very few 'Congress members'. That probably has less to do with formality or familiarity than with the other meanings of 'member', 'congress', and 'diet'. 'Diet member' is very clear (assuming you live in a country with such a legislature), but 'Congress member' or 'Congress sessions' could lead to snickers and jokes even though people perfectly well knew what you meant.