Factors that have speeded the process of phonetic changes in English
The rich vocalic system of English
Historically vowels have been subject to more changes than consonants
A modification of vowel quality results from very slight changes of the tongue or lip position
Diphthongs are the least stable
The sweeping systemic changes at the earlier periods of English which shook its sound system to the core
The lasting period of a foreign domination in the Middle Ages when the phonological system of English was under a strong influence of an alien phonological system (French)
The exclusive purity of the classic RP has been diluted, as some features of regional types of speech are "received" now, though 50 years ago those features were considered to be regional non-RP