There are a number of reasons.
The first one is that the Russian state did not prepare it's own public for this at all. If you watched, as I did, Russian television before the invasion, people such as television host Vladimir Solovyov and others, who are regarded as Kremlin propagandists, were all saying it was rubbish. They were actually laughing at it and saying that this is western propaganda, this is a load of garbage and nothing of the sort was going to happen. There was zero attempt to prepare the Russian population for it. In fact, the Russian population was told that the idea this was going to happen was rubbish.
Those of us who expected that you would prepare your population for war were thrown off a little bit by this. We were also thrown off by the fact that what you might call the top political analysts in Russia, people who know Putin, like Fyodor Lukyanov, and others, Andrey Kortunov, Dmitri Trenin—I can list off a whole pile of names—all said it wasn't going to happen. They all said it was just coercive diplomacy.
Because the top names, who supposedly know what people in the Kremlin are thinking, thought it wasn't going to happen, that convinced many of us that it wasn't going to. All the signs coming out of Moscow were against, so that's what led to this misinterpretation.