In Korean, the sound changes between words follow a highly regular and predictable pattern. In Japanese, while some have tried to make rules regarding sound changes, the fact remains that there is no discernable hard and fast rule. It is easy to say that /t/ changes to /d/ and /k/ changes to /g/ etc. when in a secondary position; this is the simple thing often said to learners of the language. However, it is a little like the /i/ before /e/ except after /c/ rule -- except when … - and there you have it -- except when. In Japanese there are lots of exceptions, so many in fact that whole theses have been written on this very topic.
Suffice it to say, I am no wiser than the next man but I do realise it is far simpler to just learn the idiomatic irregularities that appear in everyday expressions. In this case, for the average linguist bod like myself, the rule is not worth bothering with as it is just not so readily apparent or well established. Ask a Japanese and they will not have a clue -- just as much as the average Englishman will not be able to explain his utterly confusing spelling system. The smarter guy will explain about voiced and voiceless consonants but that is not perfect either.
Back to the point -- tenkan
is of Chinese origin, and of what few sensible rules there are, one is that words of Chinese origin are not subject to this sound change (except when …well, ask Mr. 中田 -- see below).
Someone above mentioned -- kotodama
. Well, consider furitama
. They also have medama
(eyeball), and mizutama
(water droplet). Then consider the case of Mr. 中田. Now is that Nakata or Nakada? Well, the only way to find out is to ask him.
Now let's think, should it be sumo-tori