I can't speak to the design criteria, as I wasn't there when the airplane was designed, but I do have time in type and experience flying and maintaining the P2V.
There's more going on than just engine, with respect to the nacelle. Remember that the nacelle is partially a fairing for the engine, and serves as the aerodynamic afterbody, but that conical shape also serves to fair additional structure, including the engine mount, oil tank, and in this case, the landing gear. Notice that the structure is greater beneath the wing than above, because the gear resides within. The angle of the nacelle afterbody is more than just the engine nacelle; it's also fairing the gear. If you follow the lines on the lower surface, after the gear doors are shut, you'll follow a consistent arc which actually takes the nacelle aft beyond the straight trailing edge of the flap. The upper surface, coming over the top of the wing, does join the wing at the trailing edge, but continues to mate with the lower lines, which extend further.
The simple answer may be the reason is the landing gear (and associated hydraulics, actuators, etc. There's also more structure and equipment contained within the afterbody and along the rear spar.
That said, as radials go, the R-3350, and the nacelle on the P2V, is quite large, and features integral air intake, rather than an external scoop as some engines have used. It's large enough that it opens in three sections, to work on the engine. It's very well designed, and represented the most advanced that radial technology reached.
The P2V was advanced, for its time, but complicated, hydraulically.
Those two jets on the wings, J-34's, avgas-burning turbojets, tripled fuel consumption when lit off, and of several different door fairings designed, turned out to be somewhat unreliable.
It's one of the few airplanes I've flown that required a crew member in the back to cut safety wire on the hydraulic lines at the varicam (variable camber horizontal stabilizer) and simultaneously swap valves on the main and sub-main hydraulic systems, at the same time the pilot in the cockpit swapped the control at the back of the pedestle in the cockpit. Not a big problem with a crew of 11, but in modern times, with a crew of two, a potentially big, big problem.
Off-topic, I know. The airplane was a workhorse.