SE210,
I think you'll find the reason is that both windshear and terrain avoidance maneuvers aim to give you the maximum rate of climb over a short duration as you are trying to overcome short term problems, e.g windshear or terrain ahead.
Vx and Vy give you best angle and rate of climb at a constant airspeed and therefore give you the best PROLONGED angle or rate.
You can, however, trade speed for height and achieve a greater angle and rate of climb, but only as long as you have airspeed to trade. THAT is why you pitch up to stick-shaker (or PLI in aircraft so equipped), on the basis that this should give you enough short duration "Zoom" to clear the terrain or maintain altitude through the windshear. If you still need to climb after that, obviously you can't pitch up any more as you'll stall, and pitching down will cause a short-duration decrease in performance, so all you can do is the best that stick-shaker speed will allow.