That is quite a large area. I imagine you have exceeded the allowable download size, probably by several orders of magnitude. I tried reducing the bounds considerably and the URI then works.
The API is provided for the benefit of people contributing to the map, not as a means of creating offline maps for general use.
If you try and export too large an area you should get suggestions on where might be a better place to get map data from. See the comment on this help question: