Why are two maximum power dissipation mentioned for this transistor?
They're limits for different operating conditions:



So the first limit is when the case temperature (\$T_C\$) is held at 25 C by some means (for example with a heat sink and forced air cooling, or just by operating the part with low duty cycle).

And the second one is when the ambient temperature (\$T_A\$) is 25 C, so the case will be heating up above that temperature.