Good porting means balancing and mitigating the strengths and weaknesses of each platform. If Console A has less graphical power than Console B, then THAT version might get geometrically simpler models or smaller textures or a lower framerate. If Console C has the juice to perform full physics on a moving car while Console D doesn't, your build for Console D will have a simplified physics model.

That costs a lot more money and time than just focusing all development on one platform and releasing a shoddy port for a second or third, or even less development money is not to release a port at all. Remember that unlike a PC that a great deal of development tools for each platform are platform specific, meaning that even if they don't have to scale back the graphics on a port they still have to expend a lot of effort just to get the exact same thing running at the same quality level.