My guess - the spokes on the original wheels were under-tensioned. This would effectively make the wheel/spokes absorb more road shock (i.e., the wheel acted somewhat like a spring). That would result in a softer ride, but it would cause the spokes to flex a lot. Flexing a lot causes metal fatigue and ultimate spoke failure. The fact that the new wheels ride harsher indicates to me that the spoke tension is much higher. Higher tension allows less flexing (so you would experience more transmission of vibration and shocks), but the spokes will be subjected to less flexing (and therefore will be less prone to breakage).