It's the same as older people are great at being able to remember stuff when they were young, but not so good at remembering things more recent. It's the way the brain works.

Your brain has had many opportunities to reflect on the older material in the brain making it easier to access and easier to associate emotions to said material. In addition, it's been said that your brain associates accesses to the material from the last time your accessed it which reinforces the memory further. IOW every time your access material it date stamps when you did and the next time you access it it's from that last date stamp. Every one of the prior date stamps strengthens the memory.

Now, consider how much media involves so much of your brain and how much your brain is growing during your teens and early adulthood. So many access points that involve emotions, events, etc. That's a lot of bandwidth that'll pop up when you listen to or see something that triggers again and again.