They eat too much dairy. Because they rely on the elasticity of their tissues to get their stretch. Anyone, at any age, with healthy joints, can do the full splits. B. Wearing the wrong kind of pants. The muscles tighten up and resist lengthening.
Russian scientists call it antagonist passive insufficiency.
Convince your nervous system that the new range of motion is safe. B. Convince your nervous system that it's about to be attacked. C. Convince your nervous system that it needs to work harder, for less pay. B. Try harder. C. Seek professional help.