If you have the $$ -- the Leupold Mark 4 MR/T 2.5-8x scopes were built around use on an AR. They are very nice scopes! I own an illuminated TMR model and would buy it again in a heartbeat. That being said...
A lot of people simply don't have the $1000+ to spend on optics. A lot of companies are making "tactical" models now for use on ARs. They are usually loaded with some nice features. Check out Burris -- they make nice scopes too!
Veteran OEF (2002) and OIF1 (2003) - US Army
Member of the Burnet Gun Fighters, Inc
. and of course the NRA
Oregon State University alum -- Go Beavs!