There are many opinions floating around as to why Russia is - if true - interfering in our elections. They all point to promoting the re-election of Trump. Now, there is the accusation that Sanders is unwittingly being promoted by Russia as the likely opposing candidate. This would make consistent sense only if it is thought that Sanders would be the most vulnerable. Why? - his identity with the politically taboo word in capitalist America: "socialist" - which can easily be construed by political rhetoric into "communist", as Bloomberg did in debate. In truth, the USA may not be ready for the necessary "improvements" Sanders advocates.
What are the reasons Russia (Putin) pushes for Trump's re-election? Re-hashing some of the stories from the 2016 campaign, it could be that Putin's ultimate purpose is in having someone manipulable in the White House to reducing the heavy sanctions against Russia in exchange, perhaps, for a Moscow hotel deal for Trump, or for some other quid pro quo, such as not revealing some shameful Trump behavior while in Moscow.
More likely, my guess is that shrewd Putin's strategy for retaining ludicrous Trump and his clan in the White House is that nothing short of war could do more damage to America at several levels. He is laughing offstage.