Some Democrats believed it because that's the story Clinton and Obama pushed.  I think Clinton and Obama knew it was false, and the Russians did successfully hack the election by hacking Hillary's server, and blackmailing her and Obama (who sent her emails suspiciously using an alias - and then lied he learned about her server in news reports)  because of the emails they got.  The Russians then forced them to do great favors for their ally Iran, sell US uranium to Russians, and back off the Syrian red line among other favors.  Obama did promise Russian Prime Minister Medvedev "more flexibility after the [2012] elections" - how nice to be flexible for our enemy.  It explains everything, including why the Hillary didn't get prosecuted, and her staff got immunity to destroy evidence instead of testifying about Clinton's crimes (the usual deal to get immunity).