Before Fairy, Electric had the most claim to the "light" motif so I imagine that might have been the idea, Moonlight. I think if Cresselia was in a Fairygen it'd definitely be at least part Fairy, but I think they did want it to be weak to Darkrai as well so maybe not.
If I had to guess GF played with it being part Electric because part of Cresselia's design basis are auroras which are created via an electromagnetic phenomenon; charged ion particles from space collide with atoms and molecules in the atmosphere which generate light (different atoms & molecules give off different colors of lights).
Another possibility I thought of would concern if Cresselia was created before Rotom. If Cresselia was created first, or at the very least Rotom wasn't assigned an Ability yet, in this theory GF could have already decided Cresselia's Ability being Levitate and maybe were playing with an idea of a having an Electric-type Levitate Pokemon. All the bonuses of an Electric-type, (literally) none of its weaknesses (or rather sole weakness). However, maybe wanting a more "solid" parallel between Cresselia and Darkrai (hence Cresselia not having Fairy-type even though it would fit it), Levitate would be given to Rotom.
Oh dear me, I just saw beta Simisear.
Simisear in final is a true tragedy
Gets me Simisage was a yankee delinquent at first, though final barely tweaks it. Still is the best design of the 3
Simipour...is still very plain sadly
You know, looking at beta Simisear and Simipour, I wonder if we're actually seeing the true beta Simisage.
Beta Simisear's difference is obvious, it's a big ol' chubby boy. Beta Simipour doesn't seem that different at first, but I think the length of its "dress" is more telling than we think. Final Simipour its a dress, but if you consider how short it is on beta Simipour and consider what else it could be, AND you consider they were specifically designed for Gym Leaders who were waiters... yeah, beta Simipour is wearing an eating bib. It's not a thin & tall girl, it's a snooty foodie.
So with that take, we have the food-loving chubby Simisear, the picky-eater bib-wearing Simipour... and bosozuko Simisage? I now wonder if maybe Simisage had already gone through a theme shift of separating the Elemental Monkeys from their associated Gym Leader (because while a fun visual, the Elemental Monkeys do need to stand as their own Pokemon apart from the Gym Leaders). Thinking how Simisage's tall hair could relate to a food theme, I wonder if maybe there's an original Simisage where the hair was actually more of a chef's hat (and when they decided to make them more generic, it was easier and funnier to change it to being hair like bosozuko culture). So we'd have the top chef Simisage, food critic Simipour, and general food-lover Simisear.