After several close calls with undead in very ancient times, crystal ponies burn their dead instead of burying them or leaving them in necropolises. There haven’t been large scale necromancy-related incidents since then.
That cultural practice ticked off Sombra because he’d had more slaves otherwise, and also made him much more defeatable back then because he didn’t have a horde of zombies and skeletons in addition to his brainwashed thralls.
In the crystal war timeline… well, the Crystal Empire still has no undead, but the lands Sombra’s conquering…