So?
Isn't the point of the primaries to find the candidate most people would want to vote for?
The primary should always be between two candidates or using ranked choice voting. If there are ten candidates in the primary and the winner has 10.1% of the vote you cannot tell whether they are the most popular one.
Sanders lost because he was less popular than Biden. Obama calling all other spoiler candidates to drop out ensured the candidate who most people prefer won. After all, if the candidates who dropped out had policies more similar to Sanders, they would have supported him instead of Biden.
Then why be upset Sanders lost the primary? If he didn't have a chance in the first place there is no way Biden (or anyone really) could have "stolen" his victory.