Throughout the first millenium of Japanese history (and before that even) the Japanese rather steadily expanded up Honshu. Once they had taken the whole of Honshu however they seemed largely to stop. Throughout the Edo period their dealings with Hokkaido were very limited to just small scale trading posts, there was never any serious attempt to actually expand there until the 19th century.
Why was this? I don't think the Edo period and the need to keep the clans within reach of Tokyo was the whole story as this behaviour predates that too. I've also heard the story that it was down to Japanese rice-based agriculture not working in Hokkaido...but the weather of south Hokkaido isn't all that different to north Honshu. Any ideas?