The Plus came with a keyboard in the box, unlike its newer and more expandable sister.
Also, don't forget that the Plus had another value model alongside it until September 1987--the 512Ke. At the time it made more sense to still produce the old-style keyboard since they were making two lower-end models that used it (and two higher-end models that used ADB, plus the IIGS).
What always puzzled me was why the IIGS keyboard couldn't have been used as the regular-length option on the SE. Sure, it worked, but was the designing of the Apple Keyboard (the short model most people bought with SEs) really necessary? This could have gone the other way as well if one questions the IIGS keyboard's relevancy (I think this was actually addressed in 1990 when the Apple Keyboard II came out--I seem to remember the last batch of IIGSs shipping with Apple Keyboard IIs).
Also, don't forget that the Plus had another value model alongside it until September 1987--the 512Ke. At the time it made more sense to still produce the old-style keyboard since they were making two lower-end models that used it (and two higher-end models that used ADB, plus the IIGS).
What always puzzled me was why the IIGS keyboard couldn't have been used as the regular-length option on the SE. Sure, it worked, but was the designing of the Apple Keyboard (the short model most people bought with SEs) really necessary? This could have gone the other way as well if one questions the IIGS keyboard's relevancy (I think this was actually addressed in 1990 when the Apple Keyboard II came out--I seem to remember the last batch of IIGSs shipping with Apple Keyboard IIs).