As far as DOMA is concerned, you can read about what happened here.

Here was Bill Clinton's response (from source above): "Though his official political position was against same-sex marriage, Clinton criticized DOMA as "unnecessary and divisive", while his press-secretary called it "gay baiting, plain and simple". However, after Congress had passed the bill with enough votes to override a presidential veto, Clinton signed DOMA. He claims that he did so reluctantly in view of the veto-proof majority, both to avoid associating himself politically with the then-unpopular cause of same-sex marriage, and to defuse momentum for a proposed Federal Amendment to the U.S. Constitution banning same-sex marriage. Clinton, who was traveling when Congress acted, signed it into law promptly upon returning to Washington, D.C., on September 21, 1996; he refused to hold a signing ceremony for DOMA and did not allow photographs to be taken of him signing it into law. The White House released a statement in which Clinton said "that the enactment of this legislation should not, despite the fierce and at times divisive rhetoric surrounding it, be understood to provide an excuse for discrimination, violence or intimidation against any person on the basis of sexual orientation"."

Hillary Clinton would be better off discussing precisely what conditions led to DOMA. As for 'Sanders being the only one to call her out on it' - that is inaccurate. I knew about this before your post because a separate publications wrote about the MSNBC interview. In that same interview, she was correct when she said that it was signed in to law because there was "enough political momentum" within the Republican-controlled congress. She also stated "The culture rapidly changed so that now what was totally anathema to political forces — they have ceded, they no longer are fighting except on a local level and rear guard action. And with the U.S. Supreme Court decision, it’s settled."