Why is Boston Accent non-rhotic?

From my understanding, Both British accent and American accent had been rhotic, afterward, British accent developed non-rhotic and it became prevalent in much of England while American accent has remained rhotic.

Boston, Massachusetts is one of the oldest parts of the United States but Boston Accent is non-rhotic and I don't think it makes sense.

Since Boston is the oldest city in the US, its accent should have preserved old-fashioned rhotic accent.
Even the newer parts of the US have kept rhotic and how/why did Boston accent develop non-rhotic?

Boston, New York, Philadelphia, Savannah, Charleston... These cities all had extremely close relationships with England so the dialects of these areas, to an extent, changed along with English English, while the rest of the country's speech evolved more or less independently from England's influence.

Today, non-rhoticity is considered low class, at least in Boston.

Non-rhoticity in the South has remained a feature of the "old money" in the South. It's not necessarily an upper-class thing as it is cultural.

Then why do African Americans speak non-rhotic accent?
I don't think they had extremely close relationships with England.

They didn't. Their accent is non-rhotic because of the difficulties associated with adult Africans being shipped to the US, having to learn English (or in some cases French, but that's not what we're talking about here). Their children, exposed more to their parents' speech patterns than English-speakers, which did not include the English retroflex "r." so they developed a non rhotic-accent.

>>They didn't. Their accent is non-rhotic because of the difficulties associated with adult Africans being shipped to the US, having to learn English (or in some cases French, but that's not what we're talking about here). Their children, exposed more to their parents' speech patterns than English-speakers, which did not include the English retroflex "r." so they developed a non rhotic-accent.<<

Hmm... I presumed it was because they acquired English from Southerners, for whom non-rhoticism was once far more widespread than it is today...

<<Then why do African Americans speak non-rhotic accent?
I don't think they had extremely close relationships with England. >>

No, they didn't, but they certainly did have a close relationship with the Southern aristocracy, who were more often than not English in stock, and often educated in England--hence the non-rhoticy (also hence the very English last names of many African-Americans--White, Jones, Walker, etc.).

<Today, non-rhoticity is considered low class, at least in Boston.>

Funny when you think about it, up to the 1960's on American television - non rhotic accents among women on television was quite fashionable.

Midatlantic accent (partially non rhotic) was popular then. A mixture of East Coast US accent and SOuthern British. Thank God this accent does not exist anymore.

<Today, non-rhoticity is considered low class, at least in Boston.>

Actually, the reverse is the case. It is rhoticism which is considered low-class, as it is in Commonwealth countries.

<<Actually, the reverse is the case. It is rhoticism which is considered low-class, as it is in Commonwealth countries.>>

Definitely not. It's non-rhoticism which is frowned upon or considered low-class in Boston. It's the accent of the working class.

<<Actually, the reverse is the case. It is rhoticism which is considered low-class, as it is in Commonwealth countries.>>

<<Definitely not. It's non-rhoticism which is frowned upon or considered low-class in Boston. It's the accent of the working class.>>

Guest is right: here in Massachusetts, non-rhotic accents are associated with the working class.

Rhoticism isn't seen as "low-class" in Britain. It is a feature of even the most "refined" of Scottish accents. In those areas of England where it is indelcine, it isn't because it is seen as "low-class" so much as rural and old-fashioned. Its decline in Northumberland to near extinction over the past half century hasn't been down to speakers taking up an RP accent so much as a spread of Tyneside English.