As with any other question from a future employer you answer truthfully.
Beyond the reasons stated in other answers, the recruiter may be assessing the likelihood that you will leave soon after starting. Generally speaking when companies are hiring, they want the new employee to work for them for a long time to be valuable.
If the only reason you are relocating is for the job, there is a higher likelihood that you may become unhappy with the city and choose to leave even if you like the job. If you are moving to that city for another reason, you are less likely to pack up and go as there is something else holding you there.