/u/Upthrust addressed this question as part of a larger discussion on the origins of the Security Council:

The only other country proposed for Security Council status was Brazil, at Roosevelt's urging. This was shot down quickly by the British and Soviets. The British feared Brazil would be another anti-colonial vote on the council, while the Soviets didn't want yet another Western power there.

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I should first say that while the veto was almost certainly part of how the Big Three imagined the Security Council by the time Roosevelt proposed including Brazil, I'm not sure I'd attribute Britain and Russia's rejection of Brazil to the veto. It's more likely that the Big Three recognized that they were not just selecting the members of a committee, but also recognizing the preeminent countries in the world. The UK wasn't really prepared to give Brazil that status, the USSR didn't want to feel any more outnumbered, and the only American who really pushed for Brazil was Roosevelt himself.