Why are side sticks unpopular in sailplanes

As a natural right-hander, I've often imagined adapting to a left-
side
stick
(think many powerplanes) would require far more "mental
adaptation" on my
part
than did adapting to my Zuni's right-side-stick. Maybe some
Airbus-driving glider pilots will chime in?

I have about 3500 hours in the left seat of the A-319/320. It
turned out to be no big deal with a the stick in my left hand. I also
have time in the F-16 with a right side stick. Both aircraft's sticks
are mounted vertically, so the neutral position is in the center. The
biggest issue I had was that both aircraft are "fly by wire", which
means that the stick inputs which you make go to a computer as
electrical signals, and then the computer ("HAL" in the F-16, and
"FIFI" in the A 320) gets to decide what you are trying to get the
aircraft to do, and then sends the signals on to the flight control
power units. Sometimes, things get interpreted as in a normal
aircraft, but there are occasions where one has to make somewhat
un-natural control inputs in order to get the computer to get the
aircraft to do what you want it to do....(a whole other off-topic
story)

I like flying with a side stick. It feels natural. On the Airbus, it
allows room to have a tray table for approach plates (if you don't yet
have an electronic cockpit) and meals, etc instead of having a yoke
in the way.

On the down-side, it is hard to reach across the cockpit to fly with
the other hand in case one needs to do so. Think taking a combat
hit to one arm, or other more mundane chores. A center stick
makes flying with either hand much easier.....

RO