I don't buy the Keiretsu argument.

South Korea and China both adopted the Keiretsu model for conglomerates due to Japan's Flying Geese doctrine, yet both still have fairly robust software scenes.

If I were a betting man, my hunch would be the collapse of domestic financing during the Asian Financial Crisis and Great Financial Crisis.

Japanese asset managers who concentrated on tech like SoftBank, Nomura, and MUFG had better options in Asia (South Korea, China, India) or in North America (USA) to invest in with better returns compared to Japan.

This is why SoftBank has always been a prominent checkwriter in those markets.