I talked about this some here. Essentially, codification happened at a time when pronunciation was still shifting, meaning there ended up being a pretty substantial difference between the two. English's high number of loan words is less of a factor than you'd think. Middle English is a mashup of a lot of different languages, (Mostly Norman French and Old English, but with some Norse thrown in and particular authors using Latin and Greek-esque neologisms and loan words) but is written pretty much perfectly phonetically. This, of course, leads to massive variation in spelling, even within the same author's work, as I explain in the linked answer.