The other directly attested Old Iranian dialects are the two forms of Avestan, which take their name from their use in the Avesta, the liturgical texts of indigenous Iranian religion that now goes by the name of Zoroastrianism but in the Avesta itself is simply known as vohu daena (later: behdin). The language of the Avesta is subdivided into two dialects, conventionally known as "Old (or 'Gathic') Avestan", and "Younger Avestan". These terms, which date to the 19th century, are slightly misleading since 'Younger Avestan' is not only much younger than 'Old Avestan', but also from a different geographic region. The Old Avestan dialect is very archaic, and at roughly the same stage of development as Rigvedic Sanskrit. On the other hand, Younger Avestan is at about the same linguistic stage as Old Persian, but by virtue of its use as a sacred language retained its "old" characteristics long after the Old Iranian languages had yielded to their Middle Iranian stage. Unlike Old Persian, which has Middle Persian as its known successor, Avestan has no clearly identifiable Middle Iranian stage (the effect of Middle Iranian is indistinguishable from effects due to other causes).
Why did old Persian retained its old characteristics long after the old Iranian languages (If the question is unanswerable, say "unanswerable")
unanswerable