I think the most likely explanation of the phenomenon is that fantasy is normally in a medieval setting, and when we think "medieval", we think of kings, not of theocrats or magocrats. In other words, what limits clerics and wizards is our imagination.
If you're looking for in-game explanations, I can think of three
Tradition: If the people are used to having non-mage hereditary rulers, they will stick to it - loyal mages would defend their lord against the upstarts who want to upset the "divine and eternal order", and if that's not sufficient, the neighbouring kings will come and help. This is what happened in 18th/19th century Europe, where enemy royals united to fight the French Revolution, and stomped out Poland when it gave itself a constitution. Also note how e.g. the US has only had a very limited number of people with PhDs in political sciences and economics in the highest government positions, even though it may be argued that they would be highly qualified to rule a country.
Strife and Power Balance: Hextor is not the only god, and Heironeous will do everything to ensure Hextor's clerics will fail to take over the country. Also, neighbours might be much more comfortable knowing a not-too-powerful ruler lives next doors, and their sense of self-preservation would make them ally against, and overthrow a magocracy. In other words, while clerics and mages may have positions of high influence at courts, their enemies will make sure that this never becomes too extreme, and thus, a non-magical ruler with arcane and divine magic available to all factions might suit everyone's interest.
Stability: It is not that easy to maintain a dictatorship if it's not backed by tradition and supported by the local powers, and instability is very bad for business. A mageocrat would most likely have to restrict mages in the kingdom, and a theocracy would not tolerate other religions, thus weakening the country and possibly limiting trade. Furthermore, in a mageocracy, where the right to govern comes from powerful magic, succession might be rather messy, while hereditary rule makes life much more predictable, and thus the country will be more likely to thrive.
From the 1978 Ugly-Demon printing of the 1st edition AD&D Player's Handbook:
Certain halfling characters have infravision. Those with mixed blook are assumed to have infravision which functions up to 30' distant, while those of pure Stoutish blood are able to see heat radiation variation at up to 60' (normal infravision).
I don't have my 2e PHB handy, so I can't verify that this quote I got off of a shady internet source is true for 2e. It matches the 1e text, though, so it seems likely enough.
Depending on their lineage, certain halfling characters have infravision. Any halfling
character has a 15% chance to have normal infravision (this means he is pure Stout), out
to 60 feet; failing that chance, there is a 25% chance that he has limited infravision
(mixed Stout/Tallfellow or Stout/Hairfeets lineage), effective out to 30 feet.
Best Answer
Halflings were originally called hobbits prior to a legal challenge.
According to Gary Gygax himself, it was due to a legal claim on "hobbit" by a company who purchased the merchandising rights on Tolkien's works in 1976:
Halflings were originally called "hobbits" in the Dungeons & Dragons box set first released in 1974. Later printings changed this to "halfling".
According to The Acaeum, this change occurred between the fifth printing (Dec 1975 - Apr 1976) and sixth printing (1977), with the six printing being marked the Original Collector's Edition. However, Tim Kask has said that box sets were generally thrown together with whichever printings of the books were on hand, so the distinctions between printings are inexact. The Acaeum notes at least one box set shipped having the revised booklets but no Original Collector's Edition mark.
Due to an oversight, a single reference to "hobbit" remained on page 6 of Men & Magic, which was fixed in the 2013 Deluxe reprint and PDF copies.
According to Wikipedia, J.R.R. Tolkien sold the film, stage, and merchandising rights to The Hobbit and Lord of the Rings to United Artists in 1968, who sold them in turn to Saul Zaentz in 1976. Zaents would go on to produce the 1978 Lord of the Rings animated movie.