Perhaps a better question for /r/askscience

The answer will simply have to be the size and shape of the GBR. Perhaps you've never seen a reef, but as you can see here we are talking here of large shallow areas and shoals some of which are the size of large islands forming an organised structure along the coast of australia. Waves break on each one of those shoals and highlights it in white foam, so it is easy to make out.

This is not a question for /r/AskHistorians, maybe /r/askscience or /r/space would be better subreddits. Anyway, the answer is, because it is 133 000 square miles. By contrast, Great Britain is only 89 000 square miles. It's big and very shallow, there are also 900 or so islands that are part of it. The water is relatively clear. That's why you can see it from space.