I was looking in a dictionary to find the meaning of the word: kalendae. When I came to the letter K, I saw that there are only a few words/names which starts with the letter K. I saw that there aren't many words which contain the letter K either. Why did the native speakers of Latin never use the letter K?
The letter K was common in the old Latin alphabet, but somehow it all but died out before the classical age. It may have been ousted by the letter C, but I'm not sure why that happened. Sometimes things happen for no apparent reason, arbitrary developments. Lewis & Short have this to say:
K, k, was used in the oldest period of the language as a separate character for the sound k, while C was used for the sound g. In course of time the character C came to be used also for the k sound, and, after the introduction of the character G, for that alone, and K disappeared almost entirely from the Latin orthography, except at the beginning of a few words, for each of which, also, the letter K itself was in common use as an abbreviation; thus, Kæso (or Cæso), Kalendæ (less correctly Calendæ), sometimes Karthago (or Kar.; v. Carthago); "and in special connections, Kalumnia, Kaput (for Calumnia and Caput, e. g. k. k. = calumniae causā in jurid. lang.): nam k quidem in nullis verbis utendum puto, nisi quae significat, etiam ut sola ponatur", Quint. 1, 7, 10; cf. id. 1, 4, 9.—Some grammarians, indeed, as early as Quintilian's time, thought it proper always to write K for initial C before a, Quint. 1, 7, 10.—Besides the above-mentioned abbreviations, the K is also found in KA. for capitalis, KK. for castrorum, K. S. for carus suis.