From what I could tell, it is because HTML tables have a specific structural meaning, which is among other things used by screen readers. So if used properly, a person who needs a screen reader could reliably have the contents of a table read to them, but if it's used for layout the result is useless. There seems to be enough people conscious of this that use of tables for layout has always been frowned on. Now there are better hints for screen readers, ,but tables are still useful for screen readers "out of the box."

In any case, tables are not nearly as flexible as a true responsive layout.