It's not actually that unique, you could just as well say Münster, Kiel or Rostock for example.

In general it's those areas because their local dialect is the deadest and there are usually only the tiniest remnants of local dialect left.

In the South people say some things more Southern, even if they are speaking "Standard German", they say the -ig in König like a "-ik" for example when "-ich" is considered Standard.
In Thuringia or Saxony obviously they say the vowels a bit differently, especially "A".

Originally the reason that those pronunciations are considered "Standard" obviously just has its base in prestige. Sometime in the 18th century Northern German had a better reputation, so it was set as the Standard. Before that Saxon was considered the purest German because of Luther.