Quite simple; they have really bad mobility penalties, and a peircing weapon negates most of the benefits of being in a warstrider.  (The fact that you can be targeted with extremely powerful warstrider piercing weapons without the user taking a -2 extrenal penalty is just gravy)  Warstrider on warstrider combat is usually over in less than 10 ticks, since fighting in a warstrider is pretty much suicidal if you don't have a perfect defense.<br/>