Suen was once a reasonably Toxic world, the ex-Rachni homeworld, and was a pretty pleasant place to live for the Rachni. They lived peacefully in their underground Hive system, building glorious insect cities underground and on the surface.
However the Rachni were soon enslaved by Reapers, who proceeded to set them on The League, leading to the Rachni Wars. After the Krogan pushed them back to Suen, they fought to the bitter end. In response, Krogan detonated bombs and levelled cities all over the planet.
Since the Wars, Suen has been nothing but a smoldering wreck, with craters dotting almost every inch of the planet. The League has basically left it alone, both as a sign of respect for the now-extinct Rachni, and because no race could establish a colony there, as not even Krogan enjoy living in a toxic atmosphere.