Or is it just because there's less chance of a subconscious boot to the rudder to pull the nose round?
That's a big part of the reason.
Low-timers have a tendency to look down at the landing area rather than over the nose. An underbanked turn with a bit too much back pressure slows the glider down, add in your subconscious boot . . .
In a well banked turn, the tendency is for the nose to drop a little and the glider picks up a few extra knots - untidy but safe.
Stall speed for the K21 is in the high 30s, yellow triangle speed is 49 knots. I would consider 50 knots to be a safe minimum approach speed for perfect conditions (ie dead calm, clean wings, flat airfield).