What are the possible explanations for why the democrats dominated congress from 1933-1994?

There are several reasons to support this. Back in the 1950's and 60's, the high unemployment rate equated to high Democratic backing. The democratic backing rose significantly as more and more issues surfaced. State Legislation which were controlled by the democrats redrew congressional district maps.
This made it difficult for the Republicans to win House seats. Up until the 90's till today, it is evident that democrats tend to do better in low-turnout districts as compared to the republicans who tend to do well in high-turnout district and thus, dominate congress. According to very many political science evidence, till date, suggest that electoral system has no partisan bias.