Lazily: because Stravinsky wrote it at the time he was writing his Neoclassical stuff.
The Symphony of Psalms does not have the obvious shoutouts to the Baroque or Classical period that Pulcinella or Oedipus Rex does, and in parts it sounds closer to his earlier Russian period. It certainly ostinatoes like early Stravinsky. Good catch, Anon.
(Could this be the first intelligent Anon question I’ve seen?)
But it’s certainly not as flashy as early Stravinsky: it’s somber and reserved (apart from the berserk horn in the final movement, which a friend said was a shout out to Richard Strauss), and it hews close to older understandings of liturgical music. It doesn’t fit nicely in the neoclassical Stravinsky opus, but it still fits better there than with what he was doing in 1910.