Because older albums must be better.
It could be that we are confusing "good" with "influential" and "inventive".
Look, i will try to explain...
About "because older albums must be better", this is bullsh*t... there was many classic releases in every decade so far and obviously these albums i mentioned aren't just better because they're older, jesus christ...
They are influential for prog bands even today (Haken wouldn't even exist).
They are albums that break through the PROG realm... "Holy sh*t, what this guy is talking about?"
It means that, even if they are Symphonic, Prog or Kraut rock, you don't have to like these genres to enjoy them because it's just great music...
Like, for me, IAW is the best DT release because it's the one who even people who hate DT or prog metal in general can greatly enjoy.
Summarizing, when i say one album is better than other, i'm not just talking about the enjoyment (in the case of DT, i think IAW it's the best even though i enjoy SDOIT and SFAM much more).