Why there is an Upper Explosive concentration Limit?
In simple terms, an explosion takes place when the reaction front propagates faster than the speed of sound. If it is slower, the expanding gas cloud can push away the unreacted mixture before it reaches ignition temperature.

With too much methane, the combustion reaction slows down, because the additional material takes up energy for heating, but cannot react.

A very simplistic microscopic picture would be that a hot methane molecule too often does not hit a cold oxygen molecule, with wich it could react to produce more heat, but a cold methane molecule, leaving two lukewarm methanes, than won't react any further.