The two are probably related. Intuitively, at least. For a given thrust level, a higher bypass ratio seems to imply less air going through the combustion section. Thus this smaller amount of air "works harder", probably getting hotter in the process.
I learned in college that the limiting factor in turbine efficiency is how high the turbine temp can go before it fails. Engine designers get better and better at finding ways to get the temp up. Jet engine turbine blades are really miracles of design and metallurgy.
The same improvements are being made to electric utility turbines.
"The new two-stage High Pressure Turbine section adds advanced coatings for cooling metal parts that will enable the same metal temperatures as today’s engines despite hotter gas path temperatures."
I thought it was mostly due to the much higher bypass-ratio than burning temp?