Cos a heavier car is easier to drive consistently fast.
A light car is capable of being driven faster (within good track conditions and relatively smooth track), but not many can drive it to its limit at the speeds we reach as it will be harder to drive (and setup being nailed is more critical).
Think of it by scaling up our scale. The cars reach a scale speed of 300-400mph or thereabouts, reaching top speed in less than a second, and with a scale weight of 16-18kilos = hard to drive fast!
(insane)