why aren't sailplanes equipped with ballistic parachutes?
Because of their longevity. Most were built or designed before full-airplane parachutes were a thing. When Ballistic Recovery Systems started, their initial offering was for ultralights only. This makes sense because the limited speed range of ultralights makes the parachute simple.
Higher top speeds mean that the parachute needs to unfold in stages so the forces are limited and deceleration is gradual. This is achieved with first a small extraction chute or a rocket which pulls the main parachutes from their compartment. Those are partially sewn so they unfold only partially first. Next, the stitching is torn open gradually to draw out the full opening of the main chute(s).
Compare that to a conventional parachute which is only opened when the pilot has separated from the glider and has decelerated already enough so the parachute design can be much more simple. The glider cockpits are designed for pilots wearing a chute (see CS 22.785 (c)) and the fuselage is kept as small as possible, so the retroactive installation of a much more bulky full-airplane parachute is not possible. Note that the parachute also needs to be fixed to suitably strong attachment points. This means a complete redesign of the center fuselage. Doing this to an existing glider is simply not worth it.
To incorporate a full-airplane parachute needs the development of a new parachute together with the airplane. This has happened first with the SB-13 experimental flying wing glider in 1988. You cannot simply slap an existing chute to an arbitrary glider. This glider-parachute combination now must be certified against regulations which do not yet cover full-airplane parachutes. Given that glider manufacturers are mostly small, privately held companies, most shy away from the expenses involved.
You may also say that market pull is not strong enough. If everyone would be clamouring for their own full-aircraft parachute, someone would have started to offer new designs with them. So far, the market demand simply isn't there.