Answer
The seedlings of flowering plants are very successful in large part due to the store of food in the endosperm. If the endosperm failed to develop, then the seedling would be in a very weak position, would grow slowly, and might not survive. If it did survive, it would be at a competitive disadvantage. All of these would reduce the fitness of that embryo.
Work Step by Step
To get to this answer, first consider the great role of the endosperm in making flowering plants the dominant group on Earth today: how it nourishes the seedling/embryo, speeding its growth and helping it compete. Then, if that were taken away, the embryo should have greatly reduced fitness/ability to contribute fertile descendants to the next generation.
