In this exercise, it is likely because the aim is testing of future simple specifically. 

Whenever the English is ‘will (do) ’ the preference will be to use future simple in French translation, and when it is ‘going to (do)’ the preference is going to be future proche. 

The distinction is not absolute in French (or English), but future simple is often used for more ‘distant’ future and can convey a sense of ‘intending to’ rather than definitely, whereas future proche is used for events closer in time, and more certain to occur.