The correct answer to this open question is the following.
Although there are no options attached or further context and references we can say the following.
Urbanization led to cities needing to grow in the direction of the outskirts of the city to the suburbs due to lack of space downtown. Another solution was to start to construct vertically, which means building high skyscrapers.
Sometimes the growth of the cities was not prevented either planned. The increase of job opportunities in industries and factories and the abundant public and commercial infrastructure of the cities draw many people to the cities.