I'm sick of relitigating 2016, but the answer to your question is that to progressively minded voters who actually cared about a progressive platform and not merely defeating Republicans, Clinton was not a progressive. She was a war hawk who was way too cozy with Wall Street and had an itchy finger for deregulation.
Now by very low standards she was probably the most progressive canidates to the era she was in than anybody since pre 50's (maybe FDR tbh). But that was in a sense that Obama was more progressive than Bill Clinton who was more progressive than Mondale who was probably in the ballpark of Carter etc.
The people who are part of what is understood to be the progressive movement have an idea of what that means. The people who say "I want to vote for a progressive" have a concept of what types of policies that canidate must in large part support. And if you try to change up the definition, it's just insults their intelligence.