Why isn’t parliamentary debate effective?

The opposition can only choose the topic for debate on 20 days of the parliamentary year (‘opposition days’)
Gov suffered a rare defeat on an opposition motion in 2010 when Liberal Democrats initiated a debate on the Brown government’s refusal to grant citizenship to Ghurkhas who had served in the UK armed forces.

Debates are poorly attended except at times of crisis.
In 2017, only 30 MPs turned up for a debate on the Humanitarian Crisis in Yemen and in 2016 gov held an urgent debate on Aleppo and only a handful of MPs went.