I am not sure you can ask "why" when it comes to features of natural languages. Unlike Klingon or Sindarin, they aren't designed; they evolved. The reasons for the path the evolution took (if there were any) are lost in the mists of time.
Yes English-English pronuciation often contains minefields even for English people as some words are (or were) used as class-markers or signs of greater/lesser education. It has been said that no Englishman can open his mouth and speak without another despising him.