I think that the other thing was that in the 50s and 60s, a v-drive was the only to handle big horsepower--outboards were limited to about 100-135HP, and I/Os could handle a 165HP straight six, and most of these boats would top out at 40-50 mph. When the jets came along, it was a new way to power a low profile boat with 300-400+ HP, and they could hit 60+ mph. Even in 1977, I remember a friend trying to run a I/O with a 400HP SBC, and he kept blowing the stern drives. I think that this was about the time when they started beefing up the stern drives to handle the higher HP, and since they were more fuel efficient than the jets, the I/O became the "package of choice" for new boats manufacturers.