Gregory's Server@WAHa_06x36 Jill Stein attended a media event in Moscow in 2015. She was a speaker there and attended at her own expense. Vladimir Putin sat at her table for about fifteen minutes and pleasantries were exchanged. It takes strongly motivated reasoning to leap from this to the conclusion that Jill Stein is a "Russian asset."
@gwm How many "media events" did Harris attend with Putin?
And that is far from the only reason, it is just the most blatantly visible one. Did you look at who else attended?
As for other reason, there are many: https://www.thirdway.org/memo/jill-stein-a-russian-asset-and-a-hypocrite