Disclaimer: I am not an expert in this and this is just my understanding of how to answer this question

You may or may not realize that most voters don’t usually go out well in advance and research all potential candidates, selecting the one they feel represents their values the best. Many of them don’t even check in to the conversation until the primaries are over and they can make a simple red vs. blue choice. Among voters that do participate in primaries, they mostly rely on information they learn about those potential candidates by watching advertisements, endorsements from other well known politicians, clips from debates, news and social media coverage, etc.

Creating that information (ads, debates, news coverage, social media, etc.) requires two things: money and momentum. Money comes first, and is disbursed according to the process the other commenter described-- the party talks with its donors and collectively they decide who to fund.

In Bernie’s case, he was systematically deprived of money by the DNC as described above, in addition to his moral philosophy of not taking money from big donors. Instead, he funded his campaign through small donations-- which he earned a LOT of-- but he still had fewer funds to generate advertisements, to host events, to “get the word out”.

Without this funding and support, Bernie couldn’t generate momentum as effectively. The fact that he is as popular as he is despite the lack of support from the party illustrates how popular his platform is, but that isn’t enough to get disengaged voters interested. Further, in his case, other party members actively wanted him to NOT be the nominee, so there were fewer endorsements, more intentional maneuvering by the party to convince voters to vote for other candidates, etc.

In essence, the idea that having the purest moral and policy philosophy is the most important element to winning the nomination is naive: it takes money and support from institutions, or else no one will ever even know what that pure philosophy is.