[NickE]

With a fixed robot width and fixed wheels all facing forwards, the shorter your wheelbase is, the easier turning will be. This is because the wheels have to scrub less to make the turn.

With a 4WD robot, the wheelbase is quite long, causing difficult turning.
When a 6WD dropped-center robot, only the front four or back four wheels are on the ground at any given time, effectively halving the length of the wheelbase, resulting in easier turning.