It's straightforward. No one else was interested in it. So nothing stood in the way when the different Russian tribes pushed eastward. Take this for instance: here's the Mongol Empire back in 1279.
Back then, Siberia was practically empty. There was truly no one occupying it. The Mongols had every chance to settle there themselves. Yet they chose not to. It simply wasn't something they wanted. Instead, they spent decades fighting wars over a barren, frozen stretch of nothing.
So eventually, the Russians claimed it, since it was sitting there for the taking. From there, they kept pushing all the way to Alaska. But by the 1800s, they'd already figured out the land had grown far too vast to manage effectively. That was really why they were so eager to unload Alaska. The Americans weren't interested either at first, viewing it as basically useless. The Russians tried offering Alaska to the U.S. three separate times before it was finally accepted.
And even now, the place remains almost deserted. It's still nearly untouched.