Avira,
Great points about Sheva mitzvos and לתקן עולם
Allow me to add the halacha ruled by the Rambam in hilchos melachim:
וְכֵן צִוָּה משֶׁה רַבֵּנוּ מִפִּי הַגְּבוּרָה לָכֹף אֶת כָּל בָּאֵי הָעוֹלָם לְקַבֵּל מִצְוֹת שֶׁנִּצְטַוּוּ בְּנֵי נֹחַ
To answer er,
The reason this wasn’t practiced throughout the generations was because for most of history Yidden weren’t exactly on talking terms with goyim…