Clinton at the time was the furthest left that the US as a whole would elect. The third way stuck because it actually worked.
I’m not going to make a snarky comment about how others should’ve put in more work, because I honestly don’t think it would’ve mattered (and it’d be rude). Leftism just wasn’t going to win by any viable margin. You can’t squeeze blood from a stone – when the electorate won’t go further, you have to meet them where they are. I’d love it if Sanders would’ve been elected in the 90s instead of Clinton, but that just wasn’t possible.