The end-of-text character (ETX), also known as control-C, was inappropriate for a variety of reasons, while using control-Z as the control character to end a file is analogous to the letter Z's position at the end of the alphabet, and serves as a very convenient mnemonic aid.
ðɪ ˈɛndʌvtˈɛkst kˈæɹɪktɚ ˌiːtˌiːˈɛks, ˈɔːlsoʊ nˈoʊn æz kəntɹˈoʊlsˈiː, wʌz ˌɪnɐpɹˈoʊpɹiət fɚɹə vɚɹˈaɪəɾi ʌv ɹˈiːzənz, wˌaɪl jˈuːzɪŋ kəntɹˈoʊlzˈiː æz ðə kəntɹˈoʊl kˈæɹɪktɚ tʊ ˈɛnd ɐ fˈaɪl ɪz ɐnˈæləɡəs tə ðə lˈɛɾɚ zˈiːz pəzˈɪʃən æt ðɪ ˈɛnd ʌvðɪ ˈælfəbˌɛt, ænd sˈɜːvz æz ɐ vˈɛɹi kənvˈiːnɪənt nɛmˈɑːnɪk ˈeɪd.