Is there any indication why the old capital of the Republic is not the capital of the New Republic?
(at least in the film I saw nothing there so I hope that the novelization has something there?)
Visual Dictionary says it's because it was, to put it short, more democratic.
"To demonstrate that the New Republic was not doomed to repeat the errors of the past, one of Chancellor Mon Mothma's first edits was the restoration of the Galactic Senate, to create a forum where the Republic would define its very nature. It was decided that all worlds would have an equal say in the shaping of government. This change resulted in the campital of galactic politics moving from Coruscant, its home for millennia. Member worlds would now host the Senate on a rotating basis. At this time, it is the cosmopolitan world of Hosnian Prime where the Senate convenes."
("Hosnian Prime" insert on the "Peace Interrupted" page with Galactic map)
This tradition started right after the fall of the Empire. In Wendig's "Aftermath" book, we see the Senate convening on Chandrila (it was partly chosen because Mon Mothma was from there, presumably, but as we can see, rotated further):
“Oh!” Olia says, thrusting up a finger. “You’re going to get a shot of what’s behind us, right? Hanna City reflects the Senate’s humble new beginnings—we’re here for the people of the galaxy, all the hardworking people. And Mon Mothma is from here, so—”
Just for completeness, the novelization does NOT address the reason.