This may break the 20 year rule, but I am unsure where this question really fits.
The Shinkansen is now over 50 years old and boasts a very impressive track record. Japanese ministers always point towards its zero fatility rate, it is famous all over the world and as someone that has travelled on it from Fukuoka to Osaka and then Tokyo, it was a very enoyable ride. Yet when it comes to modern day train exports you have 3 big players from the West: Bombardier, Siemens and Alstom. Canadian, German and French. China is also trying to enter the race as is visible in recent biddings in the Southeastern Asian area.
However, outside of one track in Taiwan I don't remember the Japanese train technology being able to be exported anywhere. Why? Japan was able to engineer an amazing high speed train and network over half a century ago, one would think this would be a great foundation as an export hit. Yet this never happened and eventually the West caught up and Japan now seems to even lag behind.
As an example: my country Germany had no high speed rail road till 1991 and basically only since post reunification. One would think that the Japanese would have a huge technological advantage and would be able to export their trains to nations seeking to upgrade their railroad public infrastructure when it comes to high sped trains. Yet today Siemens is a global player in the international market, while no Japanese company really is part of it. This seems rather weird to me.
Why has Japan historically not been able to sell its train technology in the 20th century when, at least from my view, it had a huge advantage over the rest of the world?
I want to preface this by saying that I am not a historian, and that I cannot answer the question why Japan didn't attempt or succeed to export their trains to foreign countries.
However, I think there are some misconceptions in your comment from a practical and engineering perspective that I want to clear up that might partially answer your question or at least provide some context.
I don't think that the engineering advantage that Japanese companies had over the rest of the world in terms of building high speed trains is as big as you make it out to be.
The Shinkansen 0 series was first introduced into commercial service in 1964 and at the time had a top speed of 210 km/h. This made it the fastest commercial train in the world, though not by as wide a technical margin as you might think:
For example the DB 103 series made by Siemens which went into pre-production in 1965 could achieve a commercial top speed of just 10 km/h less than the Shinkansen, and the record for electric trains at the time was actually held by a highly modified French BB 9004 set in 1954 at 331 km/h.
While the Shinkansen 0 was certainly an impressive piece of engineering, various other companies around the world had the technical knowledge to build trains of comparable speeds.
The main reason why they didn't, mostly had to do with tracks rather than the trains running on them.
For example, despite the 103 series being designed to pull trains at 200 km/h from the start, it didn't actually do so in normal operations until 1977, since none of the tracks in Germany at the time were designed for those speeds.
The main factor that made and still makes Japanese high speed trains so fast and reliable is their rail network.
Prior the the Shinkansen trains in Japan had been running on 3ft6in (1067 mm) gauge rails. This gauge allows tighter radii and was thus very well suited for Japan's mountainous terrain, but is much less suited for high speed travel than Standard Gauge (1,435 mm)
Japan thus created a separate rail network in Standard Gauge for their high speed trains.
It is this complete separation between high speed trains and lower speed trains that is generally considered to be the most important technical factor for why Japan's high speed trains are so much more reliable than those of most other countries.
I think this answers half of your question. The companies behind the Shinkansen (primarily Hitachi) did not have a huge technical advantage compared to companies like Siemens or Alstrom.
Of course despite all that the Shinkansen trains are still pretty formidable and I cannot answer the question why they didn't get exported much.
Note:
I suspect that part of the reason might be that countries faced issues integrating the Shinkansen, designed specifically for high speed railway lines, into their existing train networks without rebuilding large parts of existing infrastructure from scratch.
Japan itself introduced a series of Mini-Shinkansen trains to run on older tracks converted to Normal Gauge. Despite running one the same tracks as the normal Shinkansen, it has a slimmer body to achieve the necessary minimum clearance to bridges, tunnels, platforms, and other train tracks on those older railway lines.
However, while this explanation does seem very plausible to me, I cannot tell you whether it actually played any role in other countries' decisions not to adopt the Shinkansen, or if Hitachi ever even made any serious effort to sell trains outside of Japan.