I'm confused by the question. Yeltsin was widely popular well before assuming the presidency of the RSFSR - in May 1990, he was chosen as the chair of the RSFSR's Presidium (with votes from reformers and conservatives angling to maintain their own power). He pushed for the 1991 referendum to authorize a presidential election (which passed handily), and he won the presidency with 57% of the vote, with the runner-up netting only a quarter of that.

Do you mean Yeltsin's actions during the 1991 coup, when US intelligence worked with Yeltsin and the RSFSR's elected government to undermine the coup leaders? Or are you referring to the USSR's central government, not the RSFSR, when you ask why he wasn't charged?