Why did nixon use the term “silent majority? ”

Nixon used the term "silent majority" to refer to what he saw as the main American opinion that supported his policies and did not agree with Vietnam war protesters.

Richard Nixon had campaigned for the presidency by appealing to what he called "The Silent Majority" of American citizens -- the everyday, middle-class, working folks who were not part of the anti-war protests that had been happening in the country.  On November 3, 1969, during his first year in office, President Nixon called on that "silent majority" in a major radio and television address.  In response to continuing war protests, Nixon urged solidarity in support of the war effort in Vietnam War effort, saying that the United States was “going to keep our commitment in Vietnam.”  He pledged that US forces would keep fighting until an honorable peace was achieved or until the South Vietnamese were able to defend themselves without US help.

Nixon's urgings did not stop the war protests.  In fact, the largest anti-war protest in US history then took place on November 15, 1969.  The Moratorium to End the War in Vietnam, which had staged teach-ins and demonstrations across the country in October, held a massive march and rally in Washington, DC, which was attended by half a million protesters.

the constitution gives congress the power to regulate commerce "among" the several states.