A wheel sends a signal, not a command
Turning a mouse wheel produces a series of small input events. The hardware reports that the wheel moved; the operating system and the active program decide what that movement should do. Early on, zooming was a natural possibility: wheel motion could change how large an image or view appeared.
But zoom is useful only when the program has something meaningful to enlarge or shrink. In a text document, for example, zooming changes the display size rather than moving to the next part of the page. Scrolling, by contrast, works across long documents, lists, and web pages: each turn advances the view through content.
A familiar gesture wins
Once programs treated wheel movement as scrolling, the gesture became predictable. Users could keep a hand on the mouse and move through a page without reaching for a scrollbar or keyboard. That made scrolling useful in far more situations than zooming, and software makers adopted it as the ordinary interpretation.
The wheel itself did not lose its ability to control magnification. Programs can still assign it that role, sometimes when a key is held down or when the pointer is over a particular area. The lasting change was a convention: the same physical motion came to mean “move through the content” unless the software gives it a more specific job.
