There might be exceptions to the statement in the title of your question, but I'm not going to quibble. The simplest reason for the lack or scarcity of word-initial ð in English (for all word classes outside of a small closed set of "function words") is because there is no regular historical source for it. In Old English, the unvoiced sounds [s f θ] are believed to have been in complementary distribution with their voiced counterparts [z v ð]; the voiced consonants only occurred when they were both recededpreceded and followed by anotherother voiced soundsounds (such as between vowels, or after a voiced consonant and before a vowel) and the voiceless consonants occurred elsewhere (before and after voiceless consonants, at the end of words, and also at the start of words). The distribution in Old English may have been different depending on the dialect, and some dialects of Middle English are known to have had voiced initial [v] and [z] at least instead of [f] and [s]. However, while a handful of words with initial /v/ in Modern English come from this source (vat and vixen) I can't find any comparable words with /z/ or /ð/.
If we take a broader look at languages, /ð/ is fairly rare as a distinct phoneme, and it is easily changed into other sounds, but there are definitely languages where it occurs at the start of more words than it does in English (for example, the aforementioned Modern Greek).