Sluggish swimmers

So my husband and I have been TTC for a year now. Two weeks ago he submitted a sample for analysis and today the results are in... Sluggish swimmers and no forward progression. The nurse told me he could see a specialist or we could move forward with artificial insemination. Can someone please explain to me why this cannot be remedied with medication or changes in diet? Why jump straight to insemination? I'm a little disheartened at this point.