Because a kundalini half-raised isn't in a natural and safe position. The only way your kundalini will stay risen permanently will be if you raise it fully. It then will not drop to the base of the spine unless something extraordinary happens. It's just like when you stop exercising, your muscles atrophy and you lose all muscle mass gained. This is a natural phenomenon that can be found in all beings, not just humans, so Satan has nothing to do with it.