Originally Posted by Toltec
Old English is very much like Frisian-Dutch apparently. I was reading only last week a linguist who had theory on the transition from Old English to Middle being largely because of the Normans. William I explicitly wanted England to stay an English speaking country and rejected the idea Norman French should become the native tongue so a gradual Norman-isation of English began removing all the difficult grammar rules and inflections that made it difficult for Norman speakers to learn. Normans would have omitted complex parts of the English language when they spoke to the English and the English would have omitted them when they spoke back to make themselves understandable.
The explanation in my old book on the history of the English language blamed it on the Danes. The basic idea was that the complicated inflection system in English was a major hindrance to communication with the Danes, whose language was otherwise very similar. This led to pressure to simplify and lose a lot of the inflection.
This was an old book, though, so I don't know how reliable it is. Both may have contributed, but it's notable that part of the birthplace of Middle English was in the Danelaw.