The answer is: because Bernie threatens big corporate interests.

[beastie boy]

75. Not necessarily.

2. There are a lot more leftists who will vote only for Bernie than there are centrists who will not vote for Bernie in the Generals. Besides, with the exception of Bernie supporters, I am not aware of any Democrat who expressed their preference to one candidate to the exception of all others. Perhaps you can provide some facts that will support your position.

Let's assume, for kicks and giggles, that Bernie wins the Primaries. The calculus that you are proposing may apply if you are talking about Democrats to the exclusion of every other voter, but it flies out the window in the general election.
Even if your assumption is taken for granted and all Democrats stand behind Sanders in the Generals, it will galvanize the opposition from the independents and centrist republicans against the leftist candidate, to either not sit the election out because they don't otherwise want to vote for Trump, or to express their opposition to the Democratic candidate they consider more extreme than Trump. There are more of them than Bernie's supporters who will not vote for anyone but Bernie.