The thermal exhaust port wasn't an oversight, but a design necessity that was deemed well enough protected, being at the end of a narrow trench under heavy enough shielding that nothing could penetrate except capital ship weaponry that the station's turbolaser towers could repel. As General Dodonna said, the station's defenses were designed against a large-scale assault. Pre-Rogue-One, Imperial engineers never expected a small snubfighter to physically penetrate the trench's shielding at the extreme far end from the port where they were weakest and survive all the laserfire from the turrets in the trench. and manage to hit a target that small.
