I can calculate the thermal resistance of the die-to-case from the picture but based on the spec and the surface areas I suspect the dominant difference is the case-to-air thermal resistance. The junction-to-case thermal resistance is specified the same (83 deg C/W) for both package types. The die to board (via the leads) thermal resistance may be the same too. That leaves the surface area of the packages as the variable. The plastic case has twice the surface area.
The total package areas are:
TO-18, 0.45 sq inches
TO-92, 0.21 sq inches
Neglecting the lead-to-board thermal resistance we have the die-to-package and package-to-air. Using 40 deg C/W for the package-to-air we get.
TO-18, 83 + 40/0.45 = 172 deg C/W, spec is 200 deg C/W
TO-92, 83 + 40/0.21 = 273 deg C/W, spec is 300 deg C/W