Because the Ultimak Rail is clamped directly to the barrel.
Look closely at my rifle. You can see the two screws just below the reflex sight. Then the bands that clamp it to the barrel are just visible under the back to "notches" at the top of the lower handguard.
I used to have a EoTech on the rail. Never had any heat issues but it was too heavy. The Trijicon is lighter and dosen't need batteries so its fits the "bomb proof always works" AK mentality better.