I believe that size is the most important factor in not making the keyboard a 76 noter. 76 keys tends to make it bulky and not so convenient to lug around on gigs.
Also the most common keyboard to obtain by those who make them is a 61 note config.
Recently the 76 note keyboard has become available. The Roland Atelier series has a 76 note lower keyboard so the piano voice can be played over a wider range of tones.
Lloyd