Physiology of tricuspid and bicuspid(mitral) valves
During embryogenesis at the level of an atrioventricular canal there is so called endocardial "cushion" tissue. This tissue bulges to the left side more than it does to the right and contribute to the fusion of the valvular ring to a one leaflet, while from the remaining part of the ring develops the second leaflet:



AVEC = atrioventricular endocardial cushions

As has been stated, the cushion tissue bulges more to the left, thus on the right side where the tricuspid valve is forming the leaflets does not "merge" and develop into three-leaflet valve.

Interestingly, this left side "suture" might sometimes fail to develop with resulting three-leaflet mitral valve.