I suspect the reason δι᾿ ὀλίγου is not broken up is because it's a fixed expression. Idioms like this will sometimes behave like a single unit, even if they're made up of multiple words. (This is purely theoretical; I'm not experienced enough composing Greek that I could say when this happens or in what expressions, I just know that it's a fairly normal linguistic phenomenon.)
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)