It appears that tactical sniper scopes are generally 10X and fixed power.  Anyone know what the reason for this is?

Here is a good overview:

www.snipercountry.com/Q&A1.htm

Available Technology! Once upon a time scope manufacturers were not able to make a variable scope that would stand up to the physical abuse that a scope sees in the military (mechanical failure of erector assy., etc) and scope fogging commonly seen in tropical climates(poor sealing of power ring, etc. Thus a fixed power scope was deemed necessary. 10X was chosen because at the time, optics quality and lack of more advanced lens coatings made that power the lowest available that still offered acceptable target recognition and more importantly, allowed use of a ranging system (Mil-Dot) for accurate long range use. This is changing, as the military is now using variable scopes from Leupold, U.S. Optics, and C-More.