So on landing you deploy flaps. Those give you added lift which means you can fly slower. Lowering the gear adds drag of course and slows you. So in a 'dirty' (gear down and flaps deployed) configuration you have extra lift and drag and angle of attack. Planes are rated for minimum and max speeds in various configurations so you don't damage things while flying. Based on a quick read it looks like the pilot deployed flaps and gear, slowed more than he probably realized, then turned off the final line to do a go around. As he was turning he picked up the gear and brought in the flaps to get the plane 'clean'. Gear up is fine, but with the flaps retracting , starting to turn, and his speed already below minimums he probably stalled nearly immediately and didn't have enough time and altitude to correct.