I think the answer to that question will be different per country. But since we see a steady global growth in BB, I think every many countries got the chance to eventually grow big.
Some countries struggle like Lubnan, a country that has been here since season 3, but still only got 8 managers.
Other countries show a very fast increase in the amount of users, like Taiwan, in season 9 they were still below 100 managers, now they already got almost 450. Scotland got just 12 users around season 9, while they are now at almost 40.
It changes per country, since in some countries are wealthier and other countries got a better basketball culture. I think most of the countries we got in BB got the potential to grow, but since I think userbase growth is exponential, the start is the hardest.