
It seems because of its restrictions it can only be picked by Monstrous characters (or perhaps specifically gnolls). Though it does say it's a monstrous deity, Yeenoghu is also considered predominantly a demon lord, and can be worshipped by anyone. See here:
From here.Not all Yeenoghu’s worshipers are gnolls, however. Any creature capable of embracing the beast within and who exults in killing, butchery, and slaughter can find a common purpose with this terrifying patron. Most nongnoll servants operate alone, performing obscene rituals in the light of the moon and eating the flesh of their living victims raw. A group of likeminded lunatics might gather and establish a small cult, but they soon draw attention, since their violent crimes escalate when gathered in groups.
It's not a necessary thing to note considering we have the all-encompassing Abyssal (war) and Abyssal (magic) deities, but I found this curious nevertheless. I am sure there are other deities similarly that have this quality. Baphomet for example is not limited to monstrous races (namely minotaurs). My personal suggestion is to change all planar deities to where they are not race-locked, but it is such a minor irrelevant thing to begin with.