He means depression caused by living within a system of exploitative economic relations which curtail peoples ability for free and authentic expression, a fundamental need of all human beings.

This makes sense but why are some people able to cope better with this system than others? Why do some get depressed and others fare okay?

What’s the reason this is happening? I’m yet to understand this..

Why are some people better at understanding technology or people? Why are some people able to do competitive bodybuilding? Why are some people geniuses? Why are some people paralyzed? Why do some people have deadly diseases or allergies?

Everyone's different, plus evolution. Left alone, the modern depressed would just die off soon enough.