I have no "official, out-of-universe answer" to offer. Here's my speculation though.
Her performance in X1 wasn't all that well received. I recall reading opinions that she didn't seem very relate-able. Many people observed that her accent was awkward & bit indistinct (in terms of where she was supposed to be from). Ditching the accent may've been to help Berry's performance seem more natural & her character more relate-able, particularly to American audiences.
She also became a far bigger star in between X1 & X2. She won the Oscar for best actress in early 2002. Probably due to this you can clearly see that they focused on her more in X2 both in the movie itself & in promoting the movie including posters etc. Having her speak in her natural accent may have been to "cash in" on the Halle Berry "brand" & her spike in fame at that time. Speaking in a quasi-African accent that didn't quite work in X1 in new a movie where she would be focused on even more probably didn't seem like a smart move by the filmmakers.
You may note that her accent loosened up even more in X3. She speaks far more casually & almost "street" with some of her pronunciation & expressions. Her character's changing accent is certainly a curiosity throughout that trilogy of films.