Because Russia has been gradually conquering territory, not all at once. They create puppet states and take over land, then regroup and rebuild before attacking again. There's a clear pattern of this with Georgia, Crimea, and now the whole of Ukraine. Each time prior, the international community wagged its finger and slapped sanctions. It's generally accepted now that enough is enough, and it ends here.
It certainly doesn't help either that Russia has been funding far right parties throughout the West who try to lift sanctions against Russia.