Languages change at vastly different rates. A new creole in its first generation of native speakers can evolve to almost unrecognisable by the second. Meanwhile, a language can be remain intelligible for millennia and many do.
Literacy, sacred texts and political stability are factors in language stability. So is the "one damned thing after another" path of history.
But to your question... I'd say English is the "outlier" or at least the example that needs explaining.
Old English isn't really English. A better name would be Saxon. It's far closer to other languages from the north sea mainland, and not at all similar to modern English. Frisian is still spoken, for example. It's basically a sister dialect to old English. Saxon was spoken by the "Anglo Saxons" who migrated to/colonized to Britain in the early mediaeval period. They barely wrote (Alfred's chronicles being a notable exception), and literacy was mostly Latin anyway. They were also a minority language for a long time.
Then the "English" (Anglo Saxons) lost political power to the normans. French came with the Norman Empire and Latin was the written & liturgical language. Various dialects of "Middle English" became "regional" languages alongside Irish/Scottish, Welsh/Cornish, some Danish, and possibly others. A lot of French got mixed in.
Eventually the Norman empire subsided or evolved into the British one, settled in England and "went native." They became the English nobility, and their posh dialect became early modern English (like Shakespeare).
Underlying a lot of this instability is the fact that Britain, Saxony and NW Europe generally was barbarian country. They (we) didn't live in cities, were borderline illiterate. Etc.
Old native traditions (ie, not Christianity) were remembered in epic songs and sagas, not books.
Languages evolve and get pushed up and down the pecking order much faster in barbarian country. Barbarian culture usually evolves much faster and more flexibly than conservative civic culture.
The parts of Europe colonized by Rome (the British outpost doesn't count) speak Latin today. None of the (more numerous) Roman conquests in "the east" speak Latin. The kept speaking Greek, Egyptian, Aramaic and such... languages that had been spoken and written in capital cities for thousands of years... even back then.