With a standard major key, the three primary chords (I, IV, and V, Tonic, Sub-Dominant, and Dominant) are all major. With the Dorian mode, the I chord was minor, the IV chord was Major, and the V chord was minor. I found (at least, my ear told me) that the VII chord (the relative major of the minor V chord) actually functioned better as a Dominant (the "drop the other shoe" chord).

A lot of tunes thought of as Dorian are actually dorian/minor hexatonic. With those, there's a very good reason for the dominant being VII.

The dorian/minor hexatonic scale on D goes D E F G A c d. Of the six notes in the scale, D F A are in a D minor chord and C E G are in a C major one. So two triads cover all the pitches you're ever going to get. You can restrict all the harmonic movement to be an oscillation from "home" (D minor) to "away" (C major) and back. For a certain kind of tonally simple tune, anyway.