From David Crystal’s book The Fight for English: How language pundits ate, shot, and left, by way of Delancey Place.
In spelling, the [English] language was assimilating the consequences of having a civil service of French scribes, who paid little attention to the traditions of English spelling that had developed in Anglo-Saxon times. Not only did French qu arrive, replacing Old English cw (as in queen), but ch replaced c (in words such as church–Old English cirice), sh and sch replaced sc (as in ship–Old English scip), and much more. Vowels were written in a great number of ways. Much of the irregularity of modern English spelling derives from the forcing together of Old English and French systems of spelling in the Middle Ages. People struggled to find the best way of writing English throughout the period. Even Caxton1 William Caxton introduced the printing press to England in 1476. didn’t help, at times. Some of his typesetters were Dutch, and they introduced some of their own spelling conventions into their work. That is where the gh in such words as ghost comes from.
Any desire to standardize would also have been hindered by the “Great English Vowel Shift, [which] took place in the early 1400s. Before the shift, a word like loud would have been pronounced ‘lood’; name as ‘nahm’; leaf as ‘layf’; mice as ‘mees’.”
“The renewed interest in classical languages and cultures, which formed part of the ethos of the Renaissance, had introduced a new perspective into spelling: etymology. Etymology is the study of the history of words, and there was a widespread view that words should show their history in the way they were spelled. These weren’t classicists showing off. There was a genuine belief that it would help people if they could ‘see’ the original Latin in a Latin-derived English word. So someone added a b to the word typically spelled det, dett, or dette in Middle English, because the source in Latin was debitum, and it became debt, and caught on. Similarly, an o was added to peple, because it came from populum: we find both poeple and people, before the latter became the norm. An s was added to ile and iland, because of Latin insula, so we now have island. There are many more such cases. Some people nowadays find it hard to understand why there are so many ‘silent letters’ of this kind in English. It is because other people thought they were helping.