Tracks are heavy, high-friction, and primarily useful in soft/muddy/slippery terrain where the weight distribution is essential to prevent sinking and slipping.
They take much more energy to move than wheels, and while on Earth-bound robots that's not so much of a problem, on a lander this is to be or not to be of the mission.
They weigh quite a bit. I can't imagine sensible tracks that wouldn't be at least several kilograms in the case of rubber, much more in the case of metal.
And as long as we don't plan missions to places that are swampy, six-wheeled design is already sufficient for when one wheel loses traction or encounters obstacle higer than its axis. The only scenario when that wouldn't suffice and tracks are superior is when the whole rover starts sinking in the soil. On Earth, where soil can be damp and loose, moved by erosion and rains tracks make sense for heavyweight, multi-ton vehicles. They make sense for much smaller vehicles like snowmobiles for material as loose as snow. But so far, we haven't discovered a planet (other than our own) where wheels would be so inferior to tracks to grant being replaced, despite the problems with tracks.