The "Sky Crane" was able to land Curiosity on Mars which weighs about 900 kg. The InSight lander weighs about 358 kg. 

Why was a new landing method designed, instead of re-using all components of a known working method? The Sky Crane could have allowed for a lot more scientific experiments on the Insight Lander, maybe it could have even navigate to a more favorable location due to a larger fuel capacity.