Bernie Sanders is successful at the primary level for the same reason Trump has been (IMO). Lack of ranked choice voting. Sanders doesn't come close to representing the majority of the Democratic party, but what he does do is get 100% of the far left. And all the more Moderate Democratic votes are spread amongst a larger number of candidates. Say the far left is 15% of America, then 30% of the Dem party (just choosing round made up numbers). In a primary, if one candidate full gets their section of the party's vote, and the other 70% is spread amongst 5 candidates, the 30% is likely to win or get second in every state, which adds up if different states like different ones of the 5 more main stream candidates. So it often comes down to how often those 5 candidates drop out, consolidating the 70% of the vote. So I'm sure there are some back room deals to get candidates that may not be polling well in future states to drop out. But while that may seem nefarious, it is pretty easy to say it is ensuring the majority of the party is represented.
With ranked choice, I think you end up with a candidate that represents more American's personally. Because the 70% of Democrats, likely don't have Sanders in their top 3 at all.