It's the same story with things like the lost Doctor Who episodes; people just didn't think this children's show from a few years back that they were never going to air again would ever attract anyone's interest and they needed the space (storing lots of stuff isn't free!) so in the bin it went.
This problem still exists today, too. How do you know what will be interesting to people in 20, 30, or 50 years time? You don't always know that, and storing everything is expensive and time-consuming. In a few decades people will be lamenting lost media that was created today.
In the case of Doctor Who it also didn't help the video tapes from that era were expensive and reusable. Lots of tapes were reused for other productions.
On the other hand, it's simply impossible to preserve everything produced. Who's going to watch all of youtube again from the beginning to search for interesting material?
This problem still exists today, too. How do you know what will be interesting to people in 20, 30, or 50 years time? You don't always know that, and storing everything is expensive and time-consuming. In a few decades people will be lamenting lost media that was created today.