It seems that Higashionna Kanryō trained Sanchin as a technique to acquire skill. Miyagi on the other hand thought there was already enough practice of kaishu-gata, but heishu-gata were lacking. He began to aim at physical education and martial arts in the modern era (=Imperial Japan) and he revised Sanchin to serve as the fundamental kata (kihon-gata) of Gōjū-ryū. And he made it into a technique of training the mind rather than to acquire skill (of which there was already enough in all the kaishu-gata).
Or in his own words:
“Looking at karate in Okinawa, I suspect that there is still a slight inclination towards neglecting the heishu-gata (form of closed techniques), but I wonder, how about that? Therefore, because the fundamental technique remained undetermined, and no matter how excellent the techniques of the kaishu-gata (form of open techniques) may be, it is inevitable that it [karate] has to be completed by taking the final step [i.e., adding more heishu-gata].”
(From: Miyagi Chōjun: Karate-dō Gaisetsu. Kihōin Shozō, 1934; pp. 5-6. Translation by AQ)
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