Why are the positive points in a V-n diagram associated with pitch maneuvers?
Because they are reached when the aircraft flies a pitch maneuver. Let's look at the three points in detail:


Point A: This is where the highest turn rate can be achieved, so it is reached normally in an attempt to change direction. You might argue correctly that this is not a clean pitch-up, but essentially it is. When starting the turn, the aircraft will deflect both aileron and rudder, but once the turn is steady, both will essentially be neutral, except for the small deflection needed to overcome damping effects. What is left is a pitch maneuver akin to a loop with mostly elevator deflection to pull the needed gs.
Point C: This is reached inadvertently when flying into a heavy gust. Aircraft fly straight for most of the time, especially when flying fast, so it is assumed that this happens in straight and steady flight. Here, not even an elevator deflection is needed to reach the envelope point, all you need to do is to fly through exceptionally turbulent weather.
Point D: This is only reached after the pilot screwed up. Certification rules like JAR 23.335 b(4)i explain what the authorities assume: The pilot went into a shallow dive when upsetting the controls, oversped the plane and now tries to slow down by pulling up. This will also most likely occur in symmetric flight only. Or maybe in the initial phase of a dogfight, but then different regulations apply.