I think the nostalgia part of the memories is that these things are new experiences at the time we experience them for the first time. Nostalgic is like the more powerful experience we had because it's the first time we really experience these new things. Over time, when we see or hear similar media, it's not that it isn't as important to us, but that we have already seen/heard similar things, so the impact is lessened. We value those original experiences more because of the power they had from being new and compare everything else to it, which fails in comparison because media changes and evolves over time.