It takes a while before we are sure that a discovery is (1) true and (2) important.

Dark energy wasn't discovered until 1998. That's pretty darn fundamental. The Standard Model wasn't really finished until the 70s--again, this is really really core stuff, and it took into the 80s before everyone was like, "wow, yeah, seems like we've really got it".

And there's oodles of still quite basic stuff being worked out. We're still wrestling with the physics of superconductivity--certainly we didn't have a good account of high temperature superconductivity before 1987 (and therefore not of the physics of superconductivity as a whole). Giant magnetoresistance was discovered in 1986(ish).

And so on.