Cracks are happening during heatup (we know this because these have widened, cooling cracks are hairlines). Assuming the cracks were not there after drying, the piece likely could have been fired successfully by employing a slower heat-up. How to solve: Smooth, compress and round those sharp concave angles. This denies cracks a place to start. Even if the concave corners could have been rounded and compressed to a tiny radius (e.g. 2mm), it would likely have helped. And rounding and compressing the sharp edges would help (there are a couple of cracks on the straight sides).
Body Cracking and Dunting During Firing
Ceramic industry can fire much faster and deal with much heavier objects than potters can, how do they do it. The answer is they pay more attention to the basics, it is all common sense and good equipment.