Nostalgia is created by reliving the past and sharing similar experience with those around you, longing for the good ol days.
Millennials (and older, maybe gen Z) lived in a time where everyone experienced the same things and not everything was recorded or easily relivable. People of the generation lived a similar and relatable experience.
There were limited channels on TV and you only had a few shows you can watch and on certain time slot. For most, if you missed it, you have to wait until next time its air. People bought their music from local stores and still had a limited selection relative to what is available today. Most of the stuff had to be in memories.
The power of nostalgia lies in our imperfect memories of the past. Nostalgia isn't about the details but the experience, it's about how the times felt instead of how it actually happened.
With the current generations now, you have streaming services like Netflix, HBO, Disney+, you can rewatch old movies anytime you want, easily accessible at the palm of your hands. You can watch most old content on Youtube somewhere with every detail. Our rose color glasses becomes alot more clear.
Digital and streaming services also means instead of consuming the content that is available at the time on the limited amount of channels you had, you can choose to watch whatever you want. Instead of listening to songs only popular enough to be on the radio, you can listen to whatever you like.
With the improvement of technology and the digital era, kids no longer have to dig through childhood memories. Everyones phone now have cameras and everything is recorded. You want to check on the past, you can check on Facebook what happened on certain date and time. You can watch any movie you want at any time and listen to any music you want at any time. Much of what I mentioned contributes to the feeling of nostalgia, many with argue those who grew up in the digital era will never experience nostalgia the same way as previous generations.