Like Google’s search filter for China, it is most likely that the BBC ran into problems while making agreements with the regime to create any such service. Google yielded by altering their search engine to meet the desires of the Chinese government. The BBC’s only option would be to transmit in Russian but that would completely annihilate the point of setting up a Belarusian service.Those seem to me like reasonable suppositions. It's a pity that the corporation apparently can't bring itself to be more open about the true situation vis-à-vis Belarusian language broadcasting. Perhaps if the BBC received more inquiries about this, some official policy statement might be made. Whether anything would actually change in practical terms as a result is another matter, of course.
It should be noted, by the way, that while the rest of the BBC is funded primarily by a licence fee, the World Service is funded by the British Foreign and Commonwealth Office.