So, this is just out of curiosity/interest, why not all the unrecorded concertos by composers from Russia, Italy, France, USA, Germany, China, Japan, etc?
I don't have a list. I'm sure it would be long. Just asking.
The answer to your question is because there is only so much any one person can do. I have a particular interest in British music and hope that others might be interested in my list.
If others wish to work on similar lists of music from other countries then that would be just fine.
Regarding the USA however, there does not appear to be a huge body of American piano concertos written by well-known or relatively well-known composers. I might point out five which have not been recorded:
1944: Roy Harris No.1
1949: Paul Creston
1950: David Diamond
1953: Roy Harris No.2
2006: Benjamin Lees No.3