Sensor size. Depth of field is based on (1) sensor size (you want big), (2) aperture (you want a low number, i.e. big opening), (3) focus distance to nearest subject, and (4) focal length (I think...).
So on a full frame sensor camera, shooting someone's portrait with an 80mm will give you very shallow DoF, on a sensor the size of an iPhone, it's probably close to infinite.
Sensor size doesn't really have an effect - it's about focal length. The reason sensor size seems to have an effect is because for the same field of view with a bigger sensor, you have a longer focal length. For the same focal length, aperture and focus distance, a larger sensor doesn't change DOF; it just adds more of the surroundings to the outside of the picture.
It's a combination of wide-angle focal length and small sensor size. It is still possible to get a natural depth of field effect by getting very close to the subject, and that effect should be more pronounced with the 56mm equivalent telephoto lens.
Because sensor size has a large factor in depth of field. The camera sensors in phones are very small. This is why you can get better depth of field with a full frame camera then you can with a crop sensor and same goes for small phone sensors.
