It is hard to know for sure what is causing this change.

First, I would say that it is not ignorance.  People have always been relatively ignorant about their own members.  Therefore, this should not be a cause of change.  So what else could be happening?  My own view is that this is caused by the increasing polarization of politics in our country. 

In recent years, our country has come to be more politically polarized than at any time in recent history.  There are fewer moderates in the country.  Things like talk radio and ideological blogs have created “echo chambers” in which people only hear opinions that conform to their own opinions.  They come to hate the other side of the political spectrum much more than before.

This has two impacts.  First, it means that they are likely to hate their member if that member is from the “wrong” party.  In the past, a person might have liked their own member for “bringing home the bacon.”  Today, they are likely to hate that member if he or she is from the wrong party.  Second, it means that they will not even be satisfied with a member from their own party unless that person is sufficiently angry at the other party.  People denounce their own representatives now as sell-outs or as something other than true members of their party if their representatives try to reach compromises with the other side.

I would argue that this growing polarization is the main source of the declining regard in which people hold their own members of Congress.