I too think that the choice of gliders in the club class is mostly due to the availability of the gliders in Argentina. In most other competitions that I've seen the gliders with handicaps around 100 are still preferred.

The D2 in the Std. class are mostly the "a" version of the fuselage which seems to have a certain impact on the performance of the glider. Might also be a coincidence though that most of the top pilots are flying a D2 instead of a LS8. It is quite obvious though that the ASW28 is not that famous in the Std. class.