Simpler, less expensive, less power, less demand, less stress.
The Grissom was blown up in one shot. It's engines couldn't provide power for the shields, for significant weapons, or anything of that sort. It's main purpose is to provide a ship with warp power and main power for sensors and instruments.
Comparatively, a system like the Consitituion Refit carries a system meant for combat. A ship like that needs to be that capable. The Oberth was never intended to be in any kind of combat situation, whereas ships like the Constitution are seen as front line warships and thus need more power, more resiliancy, and able to withstand combat stresses.
And the Excelsior was supposedly transwarp. Fail or no, the new engines are meant to power a (then) Battlecruiser/Dreadnought sized starship, which would fill that role if Starfleet went to war. This meant it should have the power to survive the beating from a D7 and hurt it enough to make it go away, stop moving, or cease to exist as one functional starship.
So adding the nacelles to this design means (in a SFB Point of view) adding auxilurary engines to increase total power output. Some mainline variants to this anyway- like the F5W with an additional frigate engine (of course, SFC doesn't reflect this).
Travesty, IMO, is when you replace the engines with oberth ones. They just aren't designed to carry the weight of a combat vessel. That's a good explaination why the Okinawa does not use Oberth engines- they aren't combat optimized.
And then there's size ratio. The Oberth is a small 'civilian' vessel with something like three phasers, not much else. This ship looks like the size of an Excelsior main hull and mounts a pair of constitution engines- a good choice, since it seems to be a stand-in cruiser. Sticking oberth nacelles on a big ship is like a shipyard giving a modern carrier oars to make it move. You need either a lot of them (oars and manpower), or bigger engines (No, no bigger oars, a bigger propultion system- ala turbines, propellers, etx.)