Hi, and welcome to the forums!
The answer to your question has been talked about quite a lot over the years, but generally as part of another discussion, and there may not be a thread dedicated to exactly why not, so it's probably time to have one which tries to capture the situation in its own right.
The reason is that Hergé himself asked that there be no new stories after his death; he felt that while others might be able to produce books as good as - or even better
than - the ones he had created, the fact was that he felt that Tintin was a part of him, and that any new books wouldn't truly be Tintin without him there to guide them.
His widow, in respect of this wish, has resisted all requests to restart the series.
Initially resistant, she almost gave in, when Casterman asked that she let Hergé's studio complete the last book, Tintin and Alph-Art
, from Hergé's notes and sketches, under the supervision of Bob De Moor, who has been assisting Hergé as the story developed.
However, she has since said that when it came time to sign the contract, she could not bring herself to do it, as she was certain that Hergé would not have wanted it. Instead, she left the book unfinished, and gradually shut down the studios and let the team go.
As things stand, it's unlikely that that situation will change in the foreseeable future; however, the possibility of a new authorized book in the distant future (2052) was aired at a public talk in 2013
, to be published just before the copyright in existing books expires.
But (as was discussed here
), while Tintin will remain in copyright in Europe until 2053, due to differences in the law elsewhere (for example in the United States of America), the series will start entering the public domain in 2025; this may bring about a change in attitude, or revisions to the current "No new books" policy, we just can't tell yet.