Urban centers tend to be more multicultural, and people have more access to cultures other than their own. Also, urban dwellers tend to use a lot of infrastructure like public transportation. This leads them to understand that a good governance, and not less of it, is a very important thing.
The higher populations are more liberal due to diversity. With more people of different backgrounds, they tend to be more tolerant of each other because they have to co-exist in order to have peace. People who live in big cities are going to have different views than people who live in rural areas due to the different lifestyles.
Urban areas tend to be more tolerant and liberal than rural areas. This is probably because there are more different kinds of people who all have to live near each other and interact with one another. In a city setting, people of all races/religions/cultures work and live in close proximity. They all realize that people of other cultures are still people who deserve rights and don't deserve to be hated. Bigotry is very inconvenient when you live in a diverse, urban community.
Highly populous regions generally have a younger and more diverse population. The conspicuous consumption excesses as well as the narrow-mindedness of the majority of Republicans and conservatives do not play well with the values and realities of living in a populous region. These factors combine to make populous regions, and regions that are more urban, typically more liberal.