I wonder why they had to erase the memories.

I think you need to make a difference here between being a 90s kid and a 90s teen. When you're born in the early 80's you'll remember all of the 90's but you were mostly a teen and would remember and like different things than little children from the same time. You'd be more into the pop culture of that time. Little children would remember the kids shows and toys and stuff like that. I was born in 1991 and I remember a lot from the 90s since almost my whole childhood and my whole time in elementary school were in that decade. My sister was born in 1988 and counts as a 90's kid too but also remembers more of the teen stuff than I do. When you're born in late 90's you'll probably remember a little less but keep one important thing in mind: With the end of 1999 all things typical for the 90's have not been destroyed. They did exist in 2000, 2001 etc. as well. Especially in Europe because the 90's kind of ended in 2005 here.

Thank you for explaining this. :D