The fashion of having decaying teeth in Britain goes all the way back to Victorian times, when having rotten teeth was a sign of wealth. Brown or Black teeth displayed your affluence and ability to pay for sugar products and confectionaires. Some high class citizens would even paint their teeth black to emphasise the effect that the sugar was having.
It's along similar lines to why it's currently fashionable to burn our skin in the Sun to show we can afford to holiday abroad. Whereas before the Industrial Revolution pale skin was preferred and was a sign of high social status. The working class worked outside so had darker skin from being exposed to the Sun, whereas the upper class spent their time indoors and had light skin.
I made up some, but not all of the above
