Why do some languages in their written form tend to follow the spoken language (e.g. Italian, Spanish) keeping the orthography "synchronized" with the phonetic evolution of the language, while other languages (e.g. English, Tibetan) are much more conservative in their written form, while the phonetics of the spoken language evolves in time?
This has to do in large extent with the measure how the language community is productive with regards to writing stuff. If you have lots of literature and written communication in general, you will be more conservative in changing the orthography because you decrease the ability to read the old records.
However I do not find the comparison with Italian or Spanish particularly appropriate as their orthography did not exactly change all that much either because of the nature of phonetic changes - e.g. Latin [j] changed to [dž], then to [ž], then to [š] and finally to [x] in Spanish. But it evolved this way in all contexts, so orthographically it could be represented in all periods by the same grapheme. There can actually be confusion, because the grapheme can be pronounced the same in certain contexts.
So the difference between Spanish/Italian and English is because in English the phonetic changes were much more context dependent, resulting in phonemes splitting into two (just like Latin /g/, which split into /g/ and /x/ in Spanish, where the latter merged with the product of latin /j/ - this was much more frequent in English).
You are referring to the different levels of phonemic orthography among languages:
- A phonemic orthography is an orthography (system for writing a language) in which the graphemes (written symbols) correspond to the phonemes (significant spoken sounds) of the language.
Languages rarely have perfectly phonemic orthographies; a high degree of grapheme-phoneme correspondence can be expected in orthographies based on alphabetic writing systems, but they differ in how complete this correspondence is. English orthography, for example, is alphabetic but highly nonphonemic.
However, the Italian and Finnish orthographic systems come much closer to being consistent phonemic representations.
While the reasons for the different levels of phonomeic orthography may vary among different languages, as far as the English language is concerned it appears that the Great Vowel Shift has something to do with it:
English orthography is highly non-phonemic. As with many languages spoken over a wide area, it would in any case be hard to construct an orthography that reflected all of the main dialects of English, because of differences in phonological systems (such as between standard British and American English, and between these and Australian English with its bad–lad split, or even between adjacent counties of Britain).
The irregularity of English spelling arises:
partly because the Great Vowel Shift occurred after the orthography was established;
partly because English has acquired a large number of loanwords at different times, retaining their original spelling at varying levels;
and partly because the regularisation of the spelling (moving away from the situation in which many different spellings were acceptable for the same word) happened arbitrarily over a period without any central plan.
However even English has general, albeit complex, rules that predict pronunciation from spelling, and several of these rules are successful most of the time; rules to predict spelling from the pronunciation have a higher failure rate.