It was probably because prog rock could not sell in the Japanese market at that time. For them, they had no choice but to do so in order to make a living. If the Internet had been available at that time and music had been distributed freely around the world as it is now, they might have been able to find a market outside of Japan where there were people who enthusiastically supported them. Unfortunately, the Internet did not exist at that time. There were no international labels to sign them. So they had to change their musical style to fit the needs of the Japanese market.