The Teletype could not move its typehead backwards, so it did not have a key on its keyboard to send a BS (backspace).
ðə tˈɛlᵻtˌaɪp kʊd nˌɑːt mˈuːv ɪts tˈaɪphɛd bˈækwɚdz, sˌoʊ ɪt dɪdnˌɑːt hæv ɐ kˈiː ˌɔn ɪts kˈiːboːɹd tə sˈɛnd ɐ bˌiːˈɛs bˈækspeɪs.