Pants are not shariah compliant because they do not hold the same amount of air that a shalwar* would hold when your blow air into it to remain afloat in turbulent waters**.
That is the only reason I can come up with.
*shalwar - baggy pants that Pakistani men usually wear.
** If you fill shalwar with air and then immerse in water, you remain afloat. Common technique used by amateur Pakistani swimmers when swimming in ponds or river. See Pic No. 20. P.S: Awesome pictures by Boston Globe. A Must See.