The word "impenetently" means almost exactly the same thing as the word "remorseless" and "unrepetenant", with very little difference even in connotation.
Could that be the reason why "impenetently" is almost never used, while "remorseless" and "unrepetenant" are relatively more commonly used words?
How many sets of words that are such close synonyms exist in the English language? Is English typical or atypical in this regard?
If two or more words mean almost the same thing, will some of them eventually win the war to be used more often while others fade into disuse?
Could the winner have something to do with the fact that "impenentently" could be more easily confused with another relatively common word, "impertinently", while "remorseless" and "unrepentant" have no false friends?
Could it be that the prefix "im-" has fallen out of favor, while the prefix "un-" and the suffix "-less" have been more popular?
Also on the language front, the English language, in practice, has very specific rules about the order in which adjectives are used in a sentence that native users almost universally follow even though they have no idea that they are doing so and they are not consciously aware that the rules exist.