Sukhasana involves hip flexion and external rotation of the thigh bones (ie crossed legs). The main limiting factor for creating ease is the need to have fluid external rotation. For many people, tightness of the groin and/or outer hips resists the ability of the thigh bone to rotate outwards and allow the knees to release downwards. So, instead of the thighs and knees releasing to the earth, they lift up towards the ribs. Add in tight hip flexors and the kinetic chain really starts to go downhill.