Because Tintin will go out of copyright in 2053 and enter the public domain. At which point anyone will be able to create and publish new Tintin comics.
And while Casterman will maintain control of the property until then, they have decided to stick to their promise to Hergé. Until one year before they don't have to anymore, and plan to publish one volume in their final year of ownership.
A rather French solution. The creative team is as yet unknown. It is quite possible they are not born yet.
Le Soir, which revealed the news in an interview with Benedict and Charlotte Gallimard Mouchart also announced a sponsorship deal for the Hergé Museum in Louvain-la-Neuve and a facsimile reproduction of the original Cigars of the Pharaoh in black and white.