From David Crys­tal, 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 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.