Another reason is genetics:
There is a gene which initially was a favourable gene to have - it helped the female to stay about the lower weight limit for breeding in times of famine. Those who had it, remained fertile, those who didn't could only breed in times of plenty.

These days in the western world, there are no times of famine, and this gene has the effect of making people put on weight and develop diabetes and actually causes fertility problems as well.

The gene causes the body to be insensitive to insulin. Some people with diabetes have it purely from bad diet, others have inherited it.