I would guess that it's because it's 9 slots further down on the Empath spell circle then Evil Eye is on the Sorcerer spell circle (which also doesn't affect undead and has been around far longer). To maintain consistency, they'd have to make it possible for both to work against undead or otherwise enhance 717 if 1108 was made to work against them. Having a level 8 spell out-perform a level 17 in that manner spell isn't something they're likely to allow.

Starchitin, the OG

A severed gnomish hand crawls in on its fingertips and makes a rude gesture before quickly decaying and rotting into dust. A gust of wind quickly scatters the dust.

While Demonology could use more combat utility... it seems Necromancy would make more sense in this particular case....

Starchitin, the OG

A severed gnomish hand crawls in on its fingertips and makes a rude gesture before quickly decaying and rotting into dust. A gust of wind quickly scatters the dust.

1108 and 717 aren't really a good comparison. While 1108 does have a chance of insta-kill, it's a relatively small chance even with a lot of lore; it's more of a side effect than the main function of the spell. I'd say it's more like the insta-kill feature of 302, 1106, and 1115. 1108 is really more for immobilizing a creature, not killing it.

717 insta-kills at discrete thresholds and only immobilizes something if you don't hit that threshold. It's a powerful spell that can instantly kill a whole room of creatures and it makes sense to have some limitations on it. 1108 is just a setup spell that has a small random chance at killing the target and can never out-perform a spell that guarantees a kill at a 175 endroll.

And besides, sorcs don't have the ability to telepathically make undead remember their past feelings like empaths apparetly do, so there isn't really a consistency issue.