Why does your sump pump smell like sewage?

Because there may be sewage in it, or worse. Call a plumber.

Floor drains and washing machine drains are often drained into sumps. Grey water from these sources will begin to smell if not regularly flushed with more water and pumped out. The best way to eliminate this is to seal the sump with a clear cover so you can view the water levels. This will also help with other potential indoor air quality issues like radon gas. I also recommend an inspection plug to allow for manipulating a stuck pump float. Also if you have flood in the basement pulling the plug will provide a quick drain for the water.

You can view a photo of a cover here...

http://www.indoor-air-health-advisor.com/radon-mitigation-system.html