The two-part structure of the St Matthew Passion is not a musical choice; it is a liturgical one. Good Friday Vespers at Bach's own St Thomas Church required a sermon lasting an hour or more roughly halfway through the service, so Bach composed the Passion in two large halves specifically to be split by it, part one closing with Judas's betrayal and part two opening after the sermon with Jesus before the high priest. A modern concert performance, freed from the sermon, usually just plays the two halves back to back with a short interval, which quietly changes the pacing of an experience Bach designed around a real interruption.
The scale of the forces Bach assembled was itself unusual for its time: two full choirs, two separate orchestras each with its own continuo group, and, in Bach's own performing forces, a third choir of boy trebles doubling the chorale melody from a gallery, so that the hymn tunes the congregation already knew from ordinary Sunday worship would ring out from a different physical location than the main musical action, as though the whole church itself were joining the story rather than merely watching it performed.
That layered spatial and liturgical design largely disappeared once the work became a concert-hall staple after its 1829 revival, performed in secular halls to seated audiences with no sermon, no gallery choir and no shared congregational memory of the chorale tunes. What survives on any modern recording is the notes Bach wrote; what is much harder to recover is the specific liturgical event those notes were written to interrupt and complete.