Why is closing the center by White generally considered a concession? Why is closing the center by White generally considered a concession? I mean, pushing e4-e5 when there are Black Pawns on d5 (and e6), or pushing d4-d5 when there are Pawns on e5 (and d6), is often considered "the second best way" to play, and something that White has to do only if necessary. I.e. vs King's Indian is considered generally a stronger opening strategy to keep the d4-e5 tension (by delaying d4-d5) until necessary. Why?
The pawn advance reduces the potential, what is done cannot be undone and Black uses the information to set up accordingly. This is probably the broadest and most general explanation. In the end, however, it may turn out to be a matter of taste or fashion.