Suits were majorly worn by the wealthy to showcase their social status. With time, normal people could afford suits which meant that wealthy men were no longer identified by suits. Suits differentiated the rich from the normal people since it was only wealthy men who could afford to buy them. When the wealthy realized that normal people could access such quality clothing, it became irrelevant for them to try to differentiate themselves from them using suits. You could dress in a suit because you want to rather than to showcase your class. This meant that it was not a must for men to wear a suit for them to show others their richness reducing their popularity.
Some men find suits uncomfortable and since suits are no longer a requirement for most jobs, men decided not to wear them. When you are not comfortable while dressed in a suit, you will choose not to wear it as it is not a must for a person to wear it. The job that a person was doing would force them to wear suits even if they were uncomfortable in them. With time, jobs adopted a new culture and suits were removed as a requirement making men choose comfortable clothes rather than persevere in a suit. Working in a place without a dress code made men stop wearing suits making them less popular.
People lacked the motivation to wear suits as most renowned celebrities chose to wear casual wear. Men tend to imitate the dressing code of renowned people and when they see them wear casual clothes often, it motivates them to adapt their style. Celebrities chose not to wear suits which made them less popular as public figures influenced their followers. You will see that since your role model rarely wears a suit, you are not encouraged to wear it. Public figures popularized casual wear which replaced the suit and men chose to follow this trend. Celebrities would be seen at public events without a suit which influenced their supporters to follow suit.
Men stopped wearing suits because they found an alternative in casual wear. The availability of these clothes made men choose them as an alternative. Men wore suits because they were of high-quality making them look smart and presentable. With the increased availability of quality casual clothes, men started wearing them as an alternative to suits as they were available at a cheaper price.
You would wear a suit to differentiate yourself from others but with time, human beings advocated for equality rather than embracing their differences. Men would wear a suit to show others that they were better and well-off. This would motivate you to wear a suit for others to see what a great person you were. However, this changed as humans chose to look at others as an equal to them making them wear suits for different reasons. Men wear suits to express themselves rather than to show off. This meant that men were free to choose what to wear which made suits less popular.