Data tables can be quite wide, and necessarily so. A single row of data needs to be kept together to make any sense in a table. Tables can flex in width, but they can only get so narrow before they start wrapping cells contents uncomfortably or just plain can’t get any narrower.
Responsive design is all about adjusting designs to accomodate screens of different sizes. So what happens when a screen is narrower than the minimum width of a data table? You can zoom out and see the whole table, but the text size will be too small to read. Or you can zoom in to the point of readability, but browsing the table will require both vertical and (sad face) horizontal scrolling.