horiz tail airfoil observations

I've been perusing Fred Thomas's Sailplane Design book lately. I am
curious, why do most of the modern sailplanes use a non-symetrical
airfoil for the horiz surface, and most of the "older" sailplanes use a
symetrical airfoil?

I was looking at a Krokus at the Pensacola airshow over the weekend,
made in the 80's, it is definately using somthing like the Wortman
71-150/30 while my glider, and Apis uses an airfoil that is not
symetrical.

Cheers,
Brad