...a long-awaited report delivered the official assessment of the US intelligence community: “The available intelligence consistently points against the involvement of US adversaries in causing the reported incidents,” read an unclassified summary.
Of course, if this were the result of some secret Russian beam weapon, "the US intelligence community" would probably want to keep our vulnerability under wraps. So, regardless of whether or not "Havana Syndrome" is the result of an attack, "the US intelligence community" is motivated to say it was not. And they're the adults in the room, they're the only authority on the matter.
This is part of why people are so receptive to conspiracy theories.