That is a fair question. I would say it is for three main reasons: temperament, look and powers presentation.
Storm's powers are tied to her personality. She is well-centered, often serene. Halle Berry was actually seen better than Alexandra Shipp in that aspect.
However. she has a dual emotional side where her anger resides. She can be quite fierce. In my opinion, that side of her originated in the books when she met the Morlocks. Shipp beat Berry on this reverse side.
I have always been a fan of character's evolution in the books, from her origin forward, and the transitions she has been allowed to experience. In the original
X-Men movie, audiences got to see the "traditional" Ororo as she is often presented on television (i.e. the original animated series, etc.) and in the comics. For the most part, Berry pulls off the look extremely well although I was not a big fan of the short hair look in later on.
Conversely, artists expanded on her look over the years, which I have really enjoyed. Additionally,
X-Men: Apocalypse comprised on the character's introduction by showing her beginnings as the orphaned street urchin but as a young adult instead of a child. I am admittedly bias liking Shipp's look more in it and
X-Men: Dark Phoenix:
Finally, I have mentioned many times on the site while talking about mutants is that I thought Berry was shortchanged by directors in her powers presentation
in the way I would expect Storm to use them.
X2 has the best scene of Berry as
the weather goddess in my opinion:
Powerful but even that scene had Berry look somewhat passive. I think Shipp has benefitted much better in how her powers were presented on-screen:
Right now, Shipp is 31. It gives her flexibility in playing a twenty-something or thirty-something Ororo. Of course, my opinion would change if an MCU introduction of the mutants would be as teenagers. That angle would coincide with the first class of gifted students Professor Xavier assembled back in the 1960's comic. A younger actress would be needed in that case.