Of all the things young people could have latched onto, it's the decade of the great recession almost no place on earth really recovered from, a head bozo in the US that's somehow popular but turned the world into his personal drone playground, while the empire continued as it did.
It was the decade every social movement failed in, and mostly retreated back to moralistic forms of liberalism, just as every equally moralistic right wing reaction grew exponentially, leading to the current issues.
It was the decade climate change was decided to be fixed through a bunch of treaties worth less than the paper they were written on.
It was the decade of the start of the return of the malaise of collapsing empires. Poverty, hunger, climate change caused disasters. War had not went away, but resistance to it did.
Plus, pop culture (the thing most of the nostalgia is directed to - the eternal "It's Christmas ~~1996~~ 2013 and my parents got me ~~a Nintendo 64~~ the new Call of Duty" of obnoxious liberals) sucked ass.
okay all of this is true but class of 09 was such a vibe