I am new to sport (2 years). Because seems so difficult to stall a
glider when intentionally practicing both straight ahead and turning
stalls I have a hard time understanding why it is such a common reason
for a tragic incident like this.

Hal,
A credible witness observed that by the time Joe realized he was in
trouble, he was too high, had a slight tailwind and had too much
energy for a normal landing. If he proceeded straight ahead he was
certainly going to overrun the runway and end up rolling into the
trees. For some reason he made a hard turn to the right followed by a
hard turn to the left. He ended up low (about 50' AGL) and slow near
midfield.

Out training teaches us to keep our speed up, but at very-low
altitudes our human nature tells us to pull back on the stick in a
turn to stay away from the ground. I didn't understand this very well
until I went up with and instructor who had me practice some slow
skidding turns (like the kind of turn we might be tempted to make from
base to final if we don't have much altitude). I found that stalling
and spin entry can be much easier and more dramatic than I thought. I
am glad you asked about this.

Mark Nyberg