The first is that the admirably consistent orthography of Old English was swept away by the Norman Conquest, and English itself was eclipsed by French for three centuries, eventually emerging with its spelling much influenced by French. English had also borrowed large numbers of words from French, which for reasons of prestige and familiarity kept their French spellings. Like most other languages with alphabetic scripts, English continues to preserve foreign spellings for loanwords, even when they employ completely exotic conventions, like the 'cz' in 'Czech'.
The second major reason is the group of linguistic changes during the period after the Conquest, including the Great Vowel Shift. For example, this resulted in 'igh' in 'night' changing from a pure vowel followed by a velar fricative to a diphthong; and 'stone' changing from two syllables to one. These changes for the most part did not detract from the rule-governed nature of the spelling system; but in some cases they introduced confusing inconsistencies, like the well-known example of the many pronunciations of 'ough' (rough, through, though, thorough, trough, plough, etc.). Most of these changes happened before the arrival of printing in England; but the printing press, rather than providing the impetus for a realignment of spelling with pronunciation, merely froze the current system and introduced further inconsistencies, partly because of the use of typesetters trained abroad, particularly in the Low Countries.
A third major reason is the English language itself. English contains 24 separate consonant phonemes and, depending on dialect, anywhere from fourteen to twenty vowels and diphthongs. English uses no diacritical marks, and makes do with the twenty-six inadequate letters of the Latin alphabet. A one to one correspondence between character and sound is not possible using that unadorned alphabet in English, which requires the use of a large number of digraphss such as th and sh.
There were also minor problems like the introduction of false etymological spellings (the 'b' in 'debt' is an attempt to link it to the Latin 'debitum', the 's' in 'island' is a misplaced attempt to link it to Latin insula instead of the correct Norse igland, and the 'p' in 'ptarmigan' has no etymological justification whatsoever). Other orthographies have not been immune to these maladies. Swedish, for example, once suffered from a fashion for 'decorative spelling'.
The English spelling system can be taught to children easily using the regularities that it does possess. Although it is undoubtedly easier to learn the spelling system of Swedish, Serbian or Swahili more quickly - all these languages have more regular and simpler systems than that of English - a literate native speaker of English generally has no difficulty with a word he or she has not seen before. However, studies have shown that dyslexia occurs more often among speakers of languages such as English whose orthography differs heavily from the phonology than speakers of languages such as Finnish or Italian where the letter-sound correspondence is more regular (see: PISA report).
Loanwords are often changed in pronunciation as a result of pressure from the spelling. A good example of this is 'ski', adopted from Norwegian (in the mid-18th century; but not common before 1900) and pronounced 'shee' till the increasing popularity of the sport after the middle of the 20th century helped the 'sk' pronunciation replace it.
Proposals for spelling reform have failed for several reasons. The spelling system is really not as bad as its critics have claimed, and causes few problems; a radical reform would be offensive to the eye and cut us off from the past; and minor reforms are hardly worth the trouble. American English spelling diverged slightly from that of British English, partly as a conscious attempt at rationalisation, partly to distance the newly-independent United States from Great Britain, but the changes are so small as to make hardly any difference, and merely make work for proof readers and sellers of spell-checking software.
The spelling of English continues to evolve. Loanwords have introduced a new quasi-Italian system of pronouncing vowels - for loanwords from any language, not just Italian - and so we have 'hindu' and no longer 'Hindoo'; and under this influence the name 'Maria' no longer rhymes with 'fire' but with 'here' in RP and other non-rhotic dialects. (This influence probably started with the introduction of many actual Italian words into English during the renaissance, in fields including music - andante, viola, forte, etc.) Advertisers introduce spellings like 'smokey' (for 'smokey bacon flavoured crisps') which they fancy is somehow smokier than 'smoky'; and 'rucsac' rather than 'rucksack', to conjure up a technical atmosphere. Since the 1970s and possibly earlier, affectionate versions of women's names that sound the same as men's names have been spelt differently: Nikki and Nicky, Toni and Tony, Jo and Joe.
The spelling of the English language annually gains publicity during May because of the popularity of a Spelling bee organized at the national level in the USA as a competition for students that are under 16 years of age.