Reasons have been endlessly discussed in this forum and are reported in many posts.
Basically it is due to regulations. Brave, Uphold and Gemini are American companies under the US law. But to operate in other countries they also have to comply with the laws of each country. Many countries have requirements that make it very difficult to implement. Other countries are simply anti crypto and put all possible barriers in the activity and target foreign companies with legal threads.
On top of all of this it has been identified several fraudulent activities that required banning users from certain countries.
Brave offers rewards. And rewards are taxable in many countries. In order to comply Brave would need to gather info from every user and declare every single reward distribution to the authorities of all those countries… This is not easy to accomplish since every country has different requirements and different procedures.
So, although you may be able to use some of the supported exchanges (Uphold, Gemini, Bitflyer) to buy and sell crypto, you may not be able to receive rewards via them.
Brave is working together with these exchanges trying to find a solution. And it is giving priority to some countries. But this is a legal process that may take months until a solution is achieved and implemented.