There were many reasons why Congressional seats were becoming safer.
A major reason for this was the increase in the size of Congressional districts. As the size of Congressional districts got larger (more people, same number of Congressional districts), it became much harder for a candidate to get name recognition. Incumbents had an advantage here because people already knew their names. Studies show that voters do not know much about Congressional races and are likely to simply vote for whichever name sounds familiar. Incumbents' names are more familiar.
Similarly, the increase in district size made advertising more important. In the old days, "retail politics" could work in such races. Districts were small and candidates or their representatives could personally meet a large percentage of the people. As districts grew, this became less possible and TV ads became more important. These are very expensive and incumbents have a great advantage in fund raising. Big donors are much more likely to give to incumbents because the donors want to curry favor with them.
These two things, brought on by the increasing size of districts, helped to make the incumbency advantage greater over the years leading up to the 1990s.