
After the House Judiciary Committee (not the full house) voted to approve impeachment, both the House and Senate Minority leaders met with Nixon to inform him that he had lost the support of enough Senate Republicans that the 2/3 majority existed. Rather than be embarrassed by the House voting to approve impeachment and the Senate voting to remove him, he chose to resign and avoid the embarrassment
Nixon was going to be removed because a) he lost the backing of his party as a whole after a tape came out of Nixon ordering the CIA to pressure the FBI into stopping their investigation into Watergate and b) he was on the side of the Senate Minority to begin with. Whereas Johnson, Clinton, and Trump had solid partisan majorities in the Senate who stood with them