On Tuesday, July 4, 2017 at 4:05:32 AM UTC-7, John B. wrote:
I was looking at photos and you tube films of bicycle racing during
the 70's and 80's and it seems that the handle bars were at a very
different position then in more recent times.

Example: Eddie Marckx time trial 1974, note the downward twist of the
handle bars. with the brake levers mounted at almost the center of the
"U" bend.
https://www.youtube.com/watch?v=vvJSB4gAq3o

During a race in 1977
https://www.youtube.com/watch?v=tEAlxGC4Kzg

In 1987, still turned down
https://www.youtube.com/watch?v=GwU7GXvbjlM

In 1990
https://www.youtube.com/watch?v=eojc3AKSWGE

in 1994
https://www.youtube.com/watch?v=VtBetiDPPfg

In 2001 what may be an intermediate position
https://www.youtube.com/watch?v=KAqIVanqbuw

In 2007 note how much flatter the bars are with the brake levers fixed
almost as an extension of the top flat section of the bars.
https://www.youtube.com/watch?v=dbB2O0SmwJw

I am wondering. From the revival of bicycle racing in 1946 until the
very late 1990's or early 2000's, some 50 years, handle bar position
amongst the top riders was very similar with the top of the bars
turned down and the bottom "drops" nearly level and then it all
changed with the top section of the bars level, or nearly so and the
drops angled downward.

What happened? Did bicycle geometry suddenly change. Did bicycles get
better? Were handlebars suddenly a different shape?

I would think it would be clear what happened. Shimano introduced the brifters and they required the bar to be angled slightly differently in order to be in the proper position for the rider to be able to use them.