There are two reasons that I'm aware of. The forward funnel served the forward boiler room. The forward boilers were decommisioned during France's refit to the Norway. They were subsequently removed and replaced with diesel engines that supply electrical power to the ship. (In her earliest years Norway was prone to power outages) When the diesels were installed a new exhaust system was required as evidenced by the "tailpipes" you now see emerging from the forward funnel.
The second reason is that after the new decks were built the sunning decks were brought closer to the airelons creating an annoying combination of exhaust and soot. The rear exhaust was re-routed upwards and supposedly higher to help alleviate this problem during her 1996 drydock following her thrilling return crossing to Le Havre.