Picking a loser for education

The FT reports today:

Students at UK grammar schools outperformed their private-sector counterparts for the first time this year, grabbing a higher proportion of top GCSE grades, results revealed on Thursday.

Yet further proof that selection is schools works (note: not all independent schools are selective)?  The figures are, of course, a little misleading as ever - many of the independent schools are now doing the tougher international GCSEs that aren't included in the table.  However, whilst it should be no surprise that grammar and independent schools do better, why do none of the major parties back selection?  Fear of losing votes or giving too much power to professional teachers and academics?  Selection has been proved to work, time and again yet no party will back it, they'd rather pick a loser.