How does it become "history" so fast?

One of the most rewarding things about historic restoration and preservation is constantly learning about new things and new places.  Last week, we learned that we have a National Postal Museum in a former Post Office building in Washington D.C.

The National Postal Museum is a Smithsonian museum in Washinton, D.C.
residing in the old Post Office building next to Union Square.
Beautiful isn’t she?  Built in 1914, she served as the city’s Post Office for 72 years.  Now she houses the Postal Museum’s exhibition spaces, research library, and store in order to achieve their goal of  “preservation, study, and presentation of postal history and philately” with “exhibits, public programs, and research”.
And she’s every bit as beautiful on the inside as she is on the outside.
But the fact that our country has an entire Smithsonian museum dedicated to preserving our philately heritage wasn’t even the most mind-boggling thing we contemplated on our walk-through of this historic building.  The thought we kept getting stuck on (and have been ever since) is:

How did we go from features like the ornate, hand-crafted ceilings commonly seen in historic buildings like the Post Office building to fiberboard drop
ceilings customary in modern buildings in just a little over 50-60 years?