Why does the Japanese basketball league, B.League, have restrictions on foreign-born players?

Like many other world sports leagues, foreign quota rules are designed to ensure that homegrown (or naturalized) talent will develop, so that the country itself can compete at a global standard. This league also has an "Asian Special Quota" which allows players from certain other Asian countries to not count against the foreign quota.

Another benefit (though rarely mentioned in official rules of leagues), the rule keeps teams with more money from "buying" a championship with skilled foreigners and maintains a better competitive balance for the entire league.

https://www.bleague.global/news/b-league-announces-expansion-of-asia-special-quota-for-the-2024-25-season