I don't recall it being that difficult to achieve. Probably the difference is in early teaching and expectations: if you learn to read music as an aid to remembering pieces that are perfected over a long time, sight-reading would be slow to develop, but if you learn it as a way to be able to play new music frequently, it will come more quickly. Probably like the difference between learning a foreign language by studying grammar and working translation exercises as opposed to on-the-fly immersion — you develop different strengths.