Part of the reason is that many (most even?) gliders don't have enough elevator authority to stall the aircraft in a well-banked turn. In my Open Cirrus I can thermal at 45 degrees with the stick full back, though the buffet shows I'm near the stall and a gust can tip the balance.
Something to explore at height I'd say.
Another reason is that a 45 degree banked turn completes the turn with the lowest possible height loss - although the glider sinks faster than at 20 degrees the amount of time in the turn is less and more than compensates.
All these are explained properly in the standard books for pre-solo glider training.