The reason is that class instances are really not first-class in Haskell. You can never import a module and choose not to import its instances, not even with

  import Module (just_one_symbol_i_need)

or to not import a couple of them. I think named instances a la PureScript let you do

  import Module (instance toJsonText, ...)

which would be great to have in Haskell, but probably breaks something deep inside instance resolution.

PureScript as a "Haskell without the mistakes" really is a great idea.