That's because we're fed the massively oversimplified idea that English was one language, spoken all over the UK, and developing in a single straight line from Old English, to Middle English, to modern English.
It's obvious that today's connected society - leading to any single language being very widespread for mutual intelligibility - bears no resemblance to the way things were many centuries ago. But we're conditioned to think in terms of our own experience until we really think about it or have it pointed out. Back then, the UK was split into many different dialects, largely consolidated later by the use of the printing press. Those dialects had so much difference in some ways, that snippets of them could sound like related-but-different languages.
(And there's very little relative difference between modern English and "middle English", which is easy for us to read, notwithstanding differences in the not-yet-standardised spelling.)
And most importantly, across history, the literary language has always been the language of the elites, the ruling class, which is often not the same language spoken by the plebs. Since the language they spoke is therefore missing from the historical record, it's sometimes open to interpretation and guesswork. Many historical linguists try to make it known that middle-to-modern English can't have come directly from the dialect of Anglo-Saxon we now call Old English, but overturning (or even clarifying) dogma from the early days of any field, against years of written encyclopedias, is very difficult.