That is a very good question, but we cannot just assume that it's because the whole population of the poor and mediocre are all depressed and suicidal deep inside. Even smart people seem to be more likely to be depressed and doubt one self. Because they tend to live more in their head and overthink things.
With the arrival of the information age I think we have produced way more smart people than ever before. The internet has become such a powerful tool to educate yourself, anybody with an cellphone has a tremendous amount of knowledge in their pocket. Too bad many people choose to not use it to it's fullest potential. So that could be one of the reasons why depression and suicide rates are climbing. I'm not sure I have not done research on this matter.
There were several polls where people tried to find a correlation between the income and happiness.
The population of US as a whole indicated that increases up to 75K/year would make them happier, but further increase would not (average US family income is about 50-53K).
Anyone earning ridiculous amounts of money will make people jealous. Coping with that jealousy would make most people miserable and depressed. Imagine parking your lambo then returning and finding someone jealous has scratched the paint work.