Kinda like most "best of" lists, things get added and then it becomes canon. And usually things get dropped off from the nearer chronological end rather than the historical end.
Sort of like this -
1001 Albums You Must Hear Before You Die
Even more amusing, they haven't updated it in 6 years...
Edit - My younger spawn talks music with me a lot and she goes back to mine things that the interwebz will tell her are great when in the course of time those things were great. She has tried chewing through Pink Floyd and Radiohead and Rush at various times, none of which she found appealing. I've mentioned to her that I have that with a lot of the classics that get paraded out as being best of the best like the Beatles - I don't get it. I get that they were talented and did chord progressions that were unique and all that, but I don't enjoy them. At all. But I do have bands that clearly were influenced by them that I enjoy. So some of it is certainly personal taste and some of it was the being there when.
If an album was revolutionary 50 years ago, everything that has come after is likely to have had some influence from it, even just a smidge. Which then lessens aspect of something being thought unique as time passes. *shrug*