Why would you make that sort of assumption about a totalitarian regime like Russia that has already been proven to be an "empire of lies"? Their "evidence" was a document that they assumed (rightly) that their useful idiots would not bother to read, as it contained content of a science experiment, not evidence of a "bioweapon". The Russians claim all the good evidence was conveniently destroyed by the United States and Ukraine, presumably using Sam Fisher from Splinter Cell and maybe Jason Bourne.
Because Under Secretary of State Victoria Nuland has already admitted to them.
U.S. Under Secy. Of State Nuland Confirms Bioweapons Labs In Ukraine
What's worse, according to this they were negotiated by then Senator Obama under then President George W. Bush.