We already know there are dozens if not hundreds on the right, but at some point we'll find out how many "Putin Puppets" are on the left (aside from Jill Stein, Wikileaks / Julian Assange, Oliver Stone, etc.) as well... Tad Devine, Bernie, Tulsi Gabbard, The Intercept, Glenn Greenwald,
Stephen F. Cohen and his wife (The Nation editor-publisher) Katrina vanden Heuvel... ?
https://twitter.com/jkfecke/status/1085631869918560256
Given the current state of the world, the Democrats probably want to field a candidate that is unquestionably NOT working to further the interests of a hostile foreign power like Russia, and there are too many to choose from (Beto, Harris, Gillibrand, etc.) for someone questionable like Bernie to be a viable option... they don't want to end up like Labour with
Jeremy Corbyn who is unable to provide any leadership on Brexit, where he won't call for a second referendum because he's more pro-Brexit than the idiots currently steering the UK off a cliff.