It all depends on the level of granularity you have selected (and the capabilities of your browser).
There are various levels of granularity with various amounts of areas to display. The more areas there are in the map, the more load it puts on your browser's computing capabilities. It can go up to a point where the browser becomes completely unresponsive.
That is why we have to set an upper limit to the zoom for each level of granularity. It is also the reason why selecting some levels of granularity will automatically zoom the map closer.
If you hit the ceiling when zooming out and want to zoom out even more, make sure to change the granularity to a lower level of detail.