Actually, I think it is a better question why Clerics do get Fox's Cunning (Int boost).
As for why not Cat's Grace, I agree that for most gods it makes more sense to boost Str or Con than Dex. Many clerics and gods just aren't as concerned about Dex. Trickery domain already has Invisibility as it's second level spell, no room for Cat's Grace there. I don't really see any other domains that it would be appropriate for.
I put it down as yet another of those strange spell list things that will always be a mystery, along with why don't Wizards and Sorcerers get Make Whole. Mending was originally a Wizard spell, all the golem fixing spells are wizard, yet Make Whole is Clerical?
In any case, the spell lists are supposed to maintain a certain level of uniqueness. Think of this as a good reason for Sorcs to take Cat's Grace if they take a buff spell.