Fluid text block – For pages that set an initial or maximum scale, this means the width property actually translates into a minimum viewport width. For example, if your layout needs at least 500 pixels of width then you can use the following markup. When the screen is more than 500 pixels wide, the browser will expand the viewport (rather than zoom in) to fit the screen...
... , which has the sometimes-unwanted side effect of preventing users from zooming in.

https://developer.mozilla.org/en-US/docs/Mozilla/Mobile/Viewport_meta_tag