Please understand that the support for directory based environments was no easy thing to implement and we were plagued by all sorts of unpleasant surprises along the way. What seemed like a trivial thing to implement caused us months of work, and we are not sure we chased out all of the demons yet. Throwing yet another complex thing like hiera into the mix until the environment support works as it should would have been a very very bad idea. (By complex here, it is more the implications of how you need to be able to configure things, how they combine, etc. than the actual hiera technology).
At the moment, the environment handling supports both legacy and directory based environments. This blocks us from performing clean up of the code until the legacy environment support has been removed. Environments also come into play on the agent and environments are reused across agent and master code when running puppet apply. The amount of code, and assumptions about how things work that permeate the code base (and the thousands of tests) was a quite substantial task to tackle, and we did reach a point were we were just about ready to reverse the work and not add the directory environment support at all as it started to threaten getting the new language features released in Puppet 4.0.
During this time, also having a discussion about additional features on top of the directory environments was simply something we had to put off.
And no, it is not a two line change somewhere. The startup order of puppet, settings, applications, faces, environments, indirections, etc. is incredibly complex. A two line change may in fact buy you an even dozen can of worms individually labeled with things like performance, caching, startup order, command line overrides, data binding API, etc. etc.
I know the next question is WHEN? I honestly cannot answer that, but I can say that this will neither happen in 3.7 nor 4.0 (that will follow short after 3.7 as it is intended to be a cleanup and removal release and a switch to the "future parser/evaluator"). The team is fully booked for this work (and on that note, I should probably mention that, yes we are hiring).