Someone elsewhere mentioned a sad reason why that likely is is because (in addition to the government of Brazil trying to keep it secret) at least with the French government there's a certain high level f trust that the money will indeed do to fixing Notre Dame. Brazil's government meanwhile is one many people find untrustworthy, so even if a billionaire were to donate money there's a good chance it'd only end up lining the pockets of Brazil's elite and never put towards helping the Amazon Rainforest.

Brazil has been keeping it a secret it just got out like 4 days ago