Rice is a complete food for us, unlike milk and fruits.
One has to observe fasting during Ekadasi and those who cannot fast avoid such food so rice is prohibited.
Notes
As per Purana, Rukma Charitra state that on Ekadasi the evil spirit Mohini enters the whole rice and into whoever consumes whole rice, the evil spirit enters into them; when the rice is broken, she flees from it and broken rice is considered half food, so one has to avoid partaking whole rice on this day.