Trying to define the nature of intelligence is probably beyond the scope of this thread. The OP was asking why people are less capable of independent thought today than they used to be.
Answer:- They are not less capable. It is just one of these perennially popular notions that people have been thinking since time began such as why are kids so badly behaved these days.
Independent thought is not always the best course of action anyway. Following the herd is a natural instant. We evolved it because it keeps us out of harm.
Your independent thinker standing laughing at the crowd all running in one direction will not be laughing when he is guy that gets eaten by the bear they were all running away from. An instinct to follow the herd serves you well at times. Children learn most of their social behaviour by observing and emulating their peers. When someone joins a new company he will often moderate his behaviour as he observes the corporate culture in order to fit in. A simple survival strategy at work in modern day life. Surviving is better than not surviving even if it means conforming.
How would society function anyway if everyone was determined to go against the flow at every opportunity and flout the laws and customs of the land.
While we are at it what is really the point of driving on the left hand side of the road? Just because everyone else does? But they are just a bunch of sheep surely?