Answer: Because apples mature more quickly than hawthorn fruit.
Explanation:
Sympatric speciation can be defined as the formation(evolution) of new species from the previously surviving ancestral species while both continue to inhabit at the same place. This is because the one of the two organism there got better resources than other.
Here, the reason the maggot fly has been contributed to the incipient speciation because apple matures more quickly as compared to that of the hawthorn fruit.
The flies feeding on apple have been selected for the rapid development.