Standard British English is wonderful to listen to and easy enough for ESL students to understand, but the UK has so many different accents, that some are really tough even for English speakers from other countries to comprehend.
American English, on the other hand, has a universality about it, which makes it easy to both comprehend and mimic. We hear it on TV and in the movies so often, anyway, and the more pronounced 'r' sound appears easier for ESL students to learn, as they can then speak more phonetically and be understood anywhere in the world.
By the way, I am neither English nor American, but a Canadian citizen who was raised in South Africa and I speak English as my mother tongue! My background has enabled me to comprehend any native English speakers with relative ease, and I am especially fascinated with the way Australians speak!