Okay, there is a lot of job opportunities in metro cities, so people from other areas often migrate to metro cities. Thus, metro cities have to face huge urbanization. This is clear that there will be a tremendous population increase in metro cities. So... the the use of vehicles will increase, and thus the traffic and pollution. So, in an effort to provide cheap, comfortable [metro is not always xD] and less polluting vehicle, metros are installed in metro cities. 
Source- Just thought myself