I suspect a big part of the answer would be the fact that Mitsubishi didn't have any direct participation in the European marketplace in 1972 or 1973.
The 'Oil Shock' at the end of 1973 had a huge effect on Japanese motorsports. It set them back for years after. So even if they had finally decided to have a go, the Oil Shock would have caused it to be cancelled.
In any case, setting up motorsports-related operations abroad was a big deal for Japanese companies in that period. I know from my own interests (focused on Nissan) that it was difficult for them to find budgets within the company. They also suffered from poor exchange rates, it was hard to find staff with language skills who were prepared to spend a long time away from home and the logistics (shipping schedules, carnets, bases for operations, reliable contacts etc etc) could be daunting for anybody tasked with setting it all up.
So this also explains a little why Nissan itself, at the end of the 60s, never wanted to venture into the world beyond that of Asia-Oceania in sports cars and prototypes. Because purely based on the results achieved by the brand at the end of that decade, the cars in the R380 family, in addition to the 381, 382 and 383, were extremely competitive, including against European-made cars (such as the Porsches and Lolas).
I think that perhaps, with a little more encouragement, some Nissan cars could have been successful in a European adventure, for example, in the Interserie.
