No, there is nothing officially expanding the list of classes eligible to take Initiate of Astilabor. Whether or not this is “absurd,” if it would be “reasonable to accept” a favored soul as Initiate of Astilabor, is something you’ll have to take up with your DM.
Plenty of Initiate feats were written allowing more than one class to potentially enter them, though none included favored soul that I can tell. Aside from cleric, paladin and ranger are the most common options. The Initiate feats are mostly a Forgotten Realms thing, so some FR-specific classes like hathran can also take some (e.g. Player’s Guide to Faerûn’s Initiate of Selûne).
As for “why [a favored soul can]not [take] this?” we can only really speculate.
For the most part, D&D 3.5e tries very, very hard to avoid naming a particular class in requirements. The only examples of it that I can think of are the Initiate feats, fighter-only feats, and the sorcerer-only Draconic Heritage feats (and even those can usually be worked around with Dragontouched). I would speculate that the reason for avoiding naming specific classes is exactly this problem—when you name a particular class, you close things off for highly-similar characters and classes. I suspect this is especially a concern when you consider that the “highly-similar” options may not have even been printed yet. And broadly speaking, D&D 3.5e treats classes as metagame concepts, bundles of class features and not known, in-game things, so such a requirement would be very odd.
As far as wondering why these exceptions exist, we can ignore fighter-only feats and the Draconic Heritage feats here, I think. The fighter-only feats exist because bonus feats are fighters’ only class feature, and it’s the only way to give them unique options. Draconic Heritage isn’t actually sorcerer-only, so for whatever reason they initially did that, they undid it by printing Dragontouched. Neither of those apply to Initiate feats, so they don’t really help us answer this question.
So why do the Initiate feats restrict things to certain classes? Well, as I already mentioned, the Initiate feats are largely a Forgotten Realms thing—and the Forgotten Realms does not treat classes as metagame bundles of class features, particularly in the case of divine classes. The Forgotten Realms, rather, goes into loving detail about exactly which sorts of divine spellcasters each deity supports. The Forgotten Realms would like to say that you can only be a, say, “paladin” if you are literally a member of one of the specific knightly orders dedicated to a deity who sponsors paladins. And we do have a lot of commentary on this from various authors—Sean K. Reynolds has been notoriously adamant on this kind of thing, for example.
So it’s entirely possible that the requirement restricts things to clerics because, at least to the minds of those who wrote it, clerics are actually different and it really only works for clerics. In other words, being a cleric is more than just a spell list, turn undead, and domains. Or, at least, for some Initiate feats, this is almost-certainly the reasoning. So is the same true of Initiate of Astilabor? Who knows—it’s at least equally plausible that the author forgot about, or didn’t know about, the favored soul, or the editor didn’t want to reference a supplement, or someone was under the impression that all Initiate feats are cleric-only (after all, most are). We will never have a sure answer to that question.
