Q. Why does the Hebrew alphabet have final forms for some letters but not for others?
A. The five letters with final forms which come at the end of a word are kaf, mem, nun, peh and tzadde. A rabbinic source makes them into a mnemonic word, min hatzofim, literally “from the watchmen” (Tosafot, Shab. 104a).
In the Jerusalem Talmud there is a suggestion that these five letters were specially chosen to have final forms because each stands for an important word. Kaf is “hand”, since the Torah was given by the hand of God. Mem is ma’amar, “utterance”, symbolic for the Divine word. Nun is ne’eman, “faithful”, since Moses is called “faithful in all My house” (Num. 12:7). Peh means “mouth”, since the Torah was spoken by the mouth of God. Tzadde is tzaddik, “righteous”, since God is called “righteous in all His ways” (Psalm 145:17).
A simpler explanation, given by the Vilna Gaon, draws our attention to the fact that ancient writings did not always show where one word ended and the next began. It was therefore useful to have an elongated form for the last letter of a word. This was relatively simple by extending the bottom bar of kaf, nun, peh and tzadde, and by turning the four sides of the mem into a square. This could not be done easily with the other letters and they therefore lacked a final form.