When a vertical film of soap solution is formed, soap particles being heavy shift to lower side of film and hence surface tension on upper part is greater than the lower part which provides the necessary force to balance the weight of film. Hence, the liquid film is stable.
But in vertical film of pure water surface tension forces are same at all the points i.e. on A' B' and C'D'. Therefore, a film of pure water will break in the absence of extra force required to balance the weight.