The Noachide movement is a movement that encourages non-Jews to follow the Seven Noahide laws---that is, the Mitzvot that are binding on Noach and his descendents (in other words, on all living humans). An apparently-typical statement from the movement would be the following (from here):
According to Jewish teaching, the Torah of Moses contains a covenant (given at Mt Sinai) binding on the Jewish people consisting of 613 commandments, and another known as the ‘Covenant of Noah’ consisting of seven commandments which is binding on all the peoples of the world who are not Jewish.
See here for more information on the movement. (It seems that the Rambam was of the opinion that Jews were obligated to try to encourage gentiles to follow the Noahide laws; this was very much a minority opinion among the sages but the contemporary Chasidic movement seems fairly well-disposed to Noahidism.)
My question is the following. When spreading awareness of the Noahide laws amongst B'nei Noach, one might think it should be equally important to spread knowledge of Bris Milah amongst B'nei Abraham who are not B'nei Yisrael. This doesn't seem to be a focus of the Noahide movement, and I presume there is a good reason for this---what is it?