1. Municipal Unions-The larger the cities the larger the government, at a certain tipping point the city worker unions become large enough to exhort a considerable impact on the local elections, and for decades the Democrats have been placating the unions for their votes
2. Urban lifestyle demographics- People who tend to like large cities also tend to be left of center politically, as cities tend to attract lefties who have grown up in and grown sick of their rural conservative lifestyle.
3. Minority voters- These have been solid democrat blocs since the 60's, but with the lurching hard left of the Dems, and the negative impact on poor people of green policies, it may be something that is up for some change.