Exactly how many delegates has he won? How many does he need to secure the nomination? How many does the current second place candidate have? Before today, Buttigieg had more delegates than Sanders.
The Democratic Party ain't the GOP. For starters, they give delegates out proportionately. Bernie Sanders winning pluralities in every state means a majority of the delegates are going to other candidates. It's not like the GOP where Trump won like 40% of the national popular vote (which was that high because a lot of the later states went overwhelmingly for him after Cruz dropped out and he was the last man standing) but was able to pull out a lot of delegates because many states such as Florida are winner-take-all in the GOP presidential primaries.