Well, after "Races of Stone" there is now a bunch of PrC rather exclusively for Gnomes and Dwarves (and Goliaths ) - but that was to be expected from a racial-focus book. I believe after "Races of Destiny" in early winter there will be some good stuff for Half-orcs, which will feature heavily in that book.
Overall, I don't really think that any specific race gets the "shaft" with regard to PrC. Half-Orcs will usually drift and qualify more easily to the fighter-PrCs (which they might have to share with other races ) in the first place, while many of the other "limited" prestige classes play to a specific race's strengths.
And - generally spoken, racial PrCs are an outgrowth of a well defined habitat and culture for any specific race. Dwarves are rather iconic dwellers in underground fortresses and staunch, stubborn fighters, so are Elves (with all the "bow and magic" hype) . Far less so for Halflings (which many settings actively try to steer away from the iconic Home-Boyrural image they tend to have ) , gnomes (besides the annoying "tinkering" image - as if the entire race was a bunch of befuddled, absent-minded technical lunatics. I have yet to see a well thought out gnomish mindset in an "official" setting ) and the "half-breeds", e.g. half-orcs and half-elves, who don't even really have a culture of their own to be iconic, but will live in human, orcish or elven settings and take their "iconic" clues from there.
And there really aren't that many possibilities for various PrCs in the barbaric society Orcs seem mostly to be stuck in in the generic settings ( notably excepting Eberron ). besides not many barbaric cultures having the resources or traditions to found and maintain a special "barbaric" PrC, even among the more numerous orcs (who tend to die young and messily in general - hence they have little time to develop or even pass on "prestige" knowledge ).
In a way it makes sense, from a world-building point of view....