Unlike the old primary radar, modern radars do not see flimsy aeroplanes too well
More likely to be the materials used in the making of the gliders to reduce weight that don't reflect the radar waves as well as they would have in the past.
We still have primary radar, its just more computer processed than it used to be. Any return that isn't moving above a specified speed gets filtered out by the computer before being put on the screen, ie weather, static returns from buildings etc...