Naina..the simple answer to your query is... a large percentage of those who vote are not qualified to judge dancing. So instead they base their decision on emotions, the performers as people .. as couples .. as actors, on the performers looks, etc.
Fortunately we have been spared the videos family background etc. in this show. However, we have been subjected to illnesses, injuries and daughters wanting to see mum or dad dance. These are all ploys to pull in the sympathy vote.
As I said...it's a money making business, nothing less and nothing more.