Sanders in my opinion is/was too far left to attract as you put it, moderate democrats and even more moderate independents who for the most part fall somewhere in-between the ideologies of today's Republican and Democratic Parties.
I would have never voted for him, he's too extreme for me. But having said that, Sanders may just be ahead of his time. Much like Goldwater back in 1964 was way too far right for most Americans, he was considered an extremist from the right. Goldwater led to a conservative movement that led to the election of Reagan and the movement of the country to the right. Sanders might be a few years ahead of his time like Goldwater was. Who knows, 10-15 years from now, Sanders political philosophy might be considered main stream as conservatism under Reagan and the two Bush's were considerate mainstream by a majority of Americans.
Even Bill Clinton more or less govern much like a moderate conservative, not a liberal and certainly not as a progressive. New movements take time to catch on, for people to come around to not viewing a new movement as being extreme.
After FDR, Truman, Eisenhower, JFK with LBJ picking up the liberal mantra, most Americans weren't ready to switch to or take another path, road to the right as far as Goldwater wanted to take the country. I don't think most Americans are ready for Sanders and his ideas to push the country far to the left after Reagan, Bill Clinton, the two Bush's. Obama tried in his first two years and most Americans said stop right there with a whopping 63 seat loss in the House and an 8 seat loss in the senate in 2010. But who knows, come 2028 or 2032, that sharp left turn onto a progressive Sanders path may be just what the country is ready for.
It's high past time that we start electing Americans to congress and the presidency who put America first instead of their political party. For way too long we have been electing Republicans and Democrats who happen to be Americans instead of Americans who happen to be Republicans and Democrats.