Very short and sweet:
Bernie, you must understand arithmetic. You must understand that you now have no chance of winning the Democratic nomination. You must understand that if it were not for your continuation of your candidacy, the remaining Democratic primaries could be cancelled. You must understand that if they are held, people will die as a result.
You can cite these facts in your statement of withdrawal. They will be appreciated. You can then get behind a drive to hold the Democratic nominating convention in whatever manner will put the fewest lives at risk. You can get behind whatever will remove the Trump death-cabal from our land. And if you do, you will be appreciated.