The 15 follows a military spec. Those specs require the parts to be manufactured to a blueprinted set of measurements to ensure compatibility. The 10 does not have that spec to follow, hence every manufacturer goes with what they feel is best. Generally there a 3 patterns.
AR10 (Armalite- the only thing that can be called an AR10, patented and proprietary).
DPMS/ SR25/LR 308- mostly interchangeable (somewhat, no guarantee because of small machining tolerances, because there is no mil spec) Knights Armament is the SR-25. The make the m110 military rifle used and DPMS patterned their rifles off of this. Many others use a version of this.
Rock River- the RRA pattern was off the original (way long ago) Bushmaster pattern prior to bushmaster being bought by the Remington group. Also a proprietary pattern
Hope this explains it