I think this falls into the various places where it reflects that you can't funnel two different resources into a single skill.
For the strike, your expending a spell from memory. 1 skill fuels 1 skill. Which is the consistent stable since 2.0
Enhanced + Spell, is now 2 skills fueling 1 skill.
This doesn't work for the same reason you can not battle or memory strike an enhanced signature spell. Because no skill needs or can benifit from more than 1 skill combined into it.
Now, as a signature spell from memory, the encantion assumes the Strike keyword, so your enhanced spell is still enhanced and still a strike, you just need to use packet delivery instead of Weapon.
It's a similar reason that a Scholar can not Elemental Manipulat an Enhanced spell from memory. While it would be awesome to throw Elemental Strike 90 "x"... it also would be expending 2 abilities into a single skill.
Now, I do think we could probably save a lot of explanations in the various skills that have this potential interaction. As most of the martial/stealth skills are ready explicitly state how they interact and what they can not interact or combine with. I think a blanket section prembling the skills explaining the "no skill can be fueled by more than 1 skill" we could probably edit down and remove a lot of the words on the skills that each explain the same concept as relevant to the skill it's repeated in. It would also make the blanket rule much easier to find instead of needing to cite the individual skills that have this explination.