Conducting an invasion would be difficult due to the fact that Japan is an island. Along with this, the Japanese are aware as to what parts of their island are most likely to be invaded. Due to geographic conditions, only some parts of this island were suitable for an invasion.
Along with this, the Allied forces would have no nearby place to retreat if their invasion was failing. An invasion of this magnitude to force Japan to surrender was estimated to cause the Allied forces millions of casualties. This is why the US government decides to use atomic bombs instead, as they believe it will save lives in comparison to how many people would die if the Allied forces did a land invasion.