OP Brimstone has a radar guidance system so it can follow its target “independently” whixh is very useful especially if its moving. Most missiles rely on the pilot / gunner guiding the missile onto the target, note if you watch the anti-tank footage (eg shooting the Russian helicopter on the ground) you’ll see the missile being guided onto the target.
My 2 cents is that the US don’t think its worth the cost/effort of having such a guided missile with a smaller warhead and they focus on manual/gps guidance with a larger warhead. The Apache can launch battlefield weapons many miles away “out of range” of enemy fire and guide them onto the target. So thats my guess as to why the US aiforce hasn’t bothered with Brimstone type missiles.
IS don’t need such weapons as they kill people with suicide bombs, cutting their heads off, throwing them off buildings, executing style with guns etc