This was explained by Brad Bird (the director of the movie) at the D23 Expo in June 2017:
Director Brad Bird revealed at D23 Expo that, with the exception of
Dash's Spencer Fox, the main cast from The Incredibles will return for
Incredibles 2.
[...]
Bird explained at the D23 Expo animation panel that they needed to
recast Dash in order to make sure his character still sounded like a
10-year-old boy. Incredibles 2 takes place a minute after the end
of the original, so Bird wanted to avoid Dash suddenly having a much
deeper voice.
Spencer was ten or eleven when the first movie was made, and now he is twenty-five: thus he doesn't sound like an ten year old anymore (which Dash is supposed to be).
FYI Spencer had already been "replaced" as Dash by Raymond Ochoa: he voiced Dash in several video games (Disney Infinity, Kinect Rush: A Disney/Pixar Adventure, ...).