The AI sees a weak target of a different religion with land in the same de jure empire and goes after it, which makes perfect sense. Even if Asturias was made harder to attack at the start (by e.g. disallowing all Muslim CBs except one-county conquests against them), at what point should this magical protection vanish? After a certain number of years? After the Umayyads get their second kingdom title? After the Umayyads form Hispania? After they overrun Aquitaine? Only if Asturias grows to a point where they have as many counties as the Umayyads? Once the Umayyads gain a certain percentage of decadence?
Just granting them a bunch of event troops also doesn't seem like a viable solution, as the AI would gleefully throw them away in their allies' wars, by attacking peasant revolters, or by launching offensive wars and getting worn down, and such event troops would also stop factions and foreign Christian realms, giving Asturias avantages in other areas as well, which doesn't seem fair to the rest of the world.
Even if there is possible to figure out a way to make Asturias invasion-proof because they survived in history it doesn't make a lot of sense, as the rest of the world (barring Charlie and, to a lesser extent, Seljuk, Sabuktigin, the Mongols, and Timur) is allowed to evolve naturally. Should it also be certain that the Karling lands in 769 stay together even if Charlie gets killed by a peasant revolt and Karloman becomes incapable? Should the Magyars be protected from other nomads until they can take over Hungary, while not being allowed to blob into the steppes because that would be ahistorical? Given how easy it is for history to go off the rails, having certain realms helped or hindered because of history would create a lot of other issues and would get to a point where it makes no sense that these realms have an advantage over the rest of the map.