Why do the most populous states in the US tend to have majority Democratic?
More populous states contain large urban areas that tend to be more diverse and have greater gaps between wealthy and poor. Democrats are often seen as "defenders of the poor" and as more "progressive" on issues of race. This was a change that occurred as a result of FDR's time in the Presidency. Also, more populous states tend to have a greater presence of organized labor which typically supports Democratic candidates. Finally, large urban areas tend to have many college educated people who consider multiculturalism and progressive economics a better way to govern. This is especially true of individuals with advanced degrees. Typically, the higher the degree attained, the more leftist a person becomes. Often divisions within a state will occur between metropolitan and rural/micropolitan areas. For example: Seattle and the coastal cities of Washington vote Democrat, but inland Washington is Republican. The same is true for Salt Lake City and the rest of Utah, the major cities of Ohio vs. rural Ohio, Chicago vs the rest of Illinois, etc. These are generalities and certainly not infallible, but this is a good place to start.