Ans: In space applications, the mass of the system plays a very crucial role in the design of any machine/device. The cost is not a big factor in space application because of bigger funding but the weight of the machinery is.
More weight = More Inertia/ More Drag = More Fuel = Bigger Fuel Tanks = Again More Weight
Fins with triangular and parabolic profiles contain less material and are more efficient than the ones with rectangular profiles. Therefore triangular and parabolic fins are used in space applications.