The answer is very simple ... WEIGHT !
All the "new" aerobatic planes (Edge 540, MXS, Corvus, Extra 330SC) are as light as possible, all use Carbon-Kevlar to make it even lighter ... to make a landing gear stay in at +/- 10g would need some serious doors/locking mechanism, and that would only make the airplane heavy. The gain in drag is not worth adding so many kilos.
simple
