There are several factors that answer this question.

The first is that there was a series of events that occurred relatively closely together that stabilized english. These are: a) the translation of the liturgy into English and its mandatory use in place of Latin in the Books of Common Prayer 1549, 1552 and 1662, b) the works of William Shakespeare, and c) the translation of the bible into english most notably the Authorized, King James Version. These works published over a period of about 60 years had major stabilizing influence on the language and defined proper usage well into the late 19th and early 20th century.

Further, most of the intelligentsia i.e. the clergy and the more intellectually inclined children of the nobility and gentry attended the Universities that is Oxford and Cambridge. This helped standardize the language. Also there were the public schools, the oldest of which, King's Canterbury founded AD 597 is the oldest secondary school in the western world. These schools along with a lively market in education also provided a stabilizing influence.

Another factor was both the Royal Court and the High Court of Parliament which brought the leading citizens lay and clerical and the representatives of the towns and shires together in one place where they spoke in the vulgar tongue.

In the last 150 years there has been the Oxford English Dictionary, which while it is largely descriptive has been an additional influence.

More recently radio and television have been a stabilizing influence which has reduced regional variation.

Lastly society tends to be fairly conservative unless acted on by external forces. A major part of the story of the English Speaking People is the fact that they have been relatively self governing since John Lackland's vassals told him they had had enough of Royal innovation.